LIFE HISTORY CHARACTERISTICS AND HABITAT QUALITY OF
FLAMMULATED OWLS (OTUS FLAMMEOLUS) IN COLORADO
By
BRIAN DWIGHT LINKHART
B.S., Colorado State University, 1981
M.S., Colorado State University, 1984
A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillment
of the requirement for the degree of
Doctor of Philosophy
Department of Environmental, Population, and Organismic Biology
2001
ii
This thesis entitled:
Life History Characteristics and Habitat Quality of Flammulated Owls (Otus flammeolus) in Colorado
written by Brian Dwight Linkhart
has been approved for the Department of Environmental, Population, and Organismic Biology
________________________________________ Dr. Carl Bock, committee chair
_________________________________________ Dr. Dave Chiszar
_________________________________________ Dr. Sharon Collinge
_________________________________________
Dr. Alexander Cruz
_________________________________________ Dr. Yan Linhart
Date ____________________________
The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly
work in the above mentioned discipline.
iii
Linkhart, Brian Dwight (Ph.D., Environmental, Population, and Organismic Biology) Life history characteristics and habitat quality of Flammulated Owls (Otus
flammeolus) in Colorado
Thesis directed by Dr. Carl E. Bock
Abstract. – I determined the life history characteristics and the components of habitat
quality in a Colorado population of Flammulated Owls (Otus flammeolus) in a 19-yr
study. The owl is a small, monogamous Neotropical migrant that nests in mature
conifer forests in western North America, and is considered sensitive by the USDA
Forest Service. Conservation planning is limited by lack of data regarding the owl’s
population dynamics and habitat requirements. I assessed population dynamics based
on density, territory fidelity, dispersal, survival, and reproduction of the owls. Most
owl territories were constant in time and space despite turnover of individuals.
Density of breeding pairs showed little annual variation. Up to 70% of territories
were occupied annually by bachelor males, suggesting that females have lower
survival. Compared to other North American owls, Flammulated Owls have a small
and unvarying clutch size, high nesting success, and a long breeding lifespan,
indicating they have a life history similar to larger owls.
Territory fidelity was male-biased, as it is with most birds, and pairs rarely
divorced. Most breeding dispersals were by females that moved one or two territories
away from their original territories. However, females whose nests failed the previous
year had lower return rates to the study area than females whose previous nests were
iv
successful. Dispersal distance may be bimodal with females dispersing longer distances
after nesting failure and shorter distances after successful nests. Females dispersed to
territories where total productivity during the study was higher than on original territories,
suggesting they assessed territory quality before dispersing.
Characteristics of high-quality breeding habitats were determined by
correlating long-term demographic parameters of owls with habitat characteristics on
their territories. Territories differed significantly in total years they were occupied by
breeding pairs and in total productivity. Availability of cavity-trees for nesting
determined where owls established territories, while forest type and structure
determined whether a territory was more often occupied by breeding pairs or bachelor
males. High-quality breeding habitat for Flammulated Owls was characterized as
mature, open stands of ponderosa pine (Pinus ponderosa) mixed with Douglas-fir
(Pseudotsuga menziesii) with sufficient cavity-trees for nesting.
v
Acknowledgements
I am grateful for all of the people who provided support for this project. First,
I thank my major advisor, Dr. Carl Bock, for his guidance, advice, and support
throughout this project. I also extend special thanks to my committee members, Dr.
Dave Chiszar, Dr. Sharon Collinge, Dr. Alexander Cruz, Dr. Yan Linhart, and Dr.
Carol Wessman.
Had it not been for the U.S. Forest Service, this project could never have
started or continued over the past 19 years. The Rocky Mountain Research Station
always has been the primary source of funding and logistical support. The Manitou
Experimental Forest proved to be a perfect location for this long-term study, offering
as natural and unmodified montane forest as might be found anywhere in the region.
My discoveries and experiences on this forest always will be part of my memories.
Several employees of the U.S. Forest Service have had vital roles in this
study. I am particularly grateful to Dr. Richard T. Reynolds, who has had a marked
effect on my career. Dr. Reynolds first offered me a job as a summer technician
assisting his raptor research projects throughout Colorado when I was an
undergraduate at Colorado State University, and he sparked my interests in research
and taught me how to become a good field biologist. Over the years his supervision,
support, encouragement, and friendship have been essential to my success. I am
indebted to his efforts.
I also thank Carl Edminster and Dr. Wayne Shepperd, who have administered
the Manitou Experimental Forest over the years, and William Knott, Ross Watkins,
vi
and Steve Tapia, who as forest caretakers always were eager to help. I especially
thank Steve Tapia, who always has gone out of his way to assist with arranging and
procuring necessary equipment and materials, helping in the field, and locating
potential volunteers. He also has been a good friend. I thank Mike Morrison and
Frank Romero, who were instrumental in providing digitized data for use in
Geographic Information Systems (GIS), and Mark Roper, who assisted with GPS
equipment. Without the friendship and advice of Suzanne Joy, who dispensed
invaluable advice on GIS matters day or night, I might still be hopelessly punching
the keyboard. Finally, Rudy King was especially helpful in assisting me with
statistical analyses and his advice was greatly appreciated.
I am very grateful to several people in the Geography Department at the
University of Colorado who provided technical skill, advice, and computer equipment
for my GIS analyses including Dr. Barbara Buttenfield, Sara Farenkopf, Jim Robb,
Chris Hanson, and Tom Dickinson. I am particularly grateful to Dr. Robin Reich,
Department of Forest Sciences at Colorado State University, for providing several
months of assistance and instruction with vegetation modeling.
A project of this duration necessitates a large number of field workers and I
am very grateful to the many individuals who offered their assistance in data
collection. Especially notable were several individuals who each volunteered over
four hundred hours, including Buzz Morrison, Scott Hiller, Patrick Leavey, and
Nicholas Tucey. The commitment of these individuals, and the extent to which they
benefited my study, cannot be understated. Also assisting with field work were Josie
Bamford, Holly Barnard, Christina Bauman, Cloud Ridge Naturalists, Sam Berry,
vii
Rob Bringuel, Michelle and Tim Connelly, Peg Coontz, Dave and Laura (Scott)
Denton, Peter Gaede, Jennifer Gass, Matt Harper, Emily Holt, Stephanie Howard,
Sam Johnson, Littleton High School students, Steve Mata, Terry and Amelia
McGlynn, William Merkle, Monica Mohr, Monica Molachy, Mark and John
Morgenstern, Nickolas Neitzel, Charlie Nemkevich, Andrew Orahoske, Summer
Orniz, Amy Ottaway, Amy Pabski, Mark Platten, Jenna Sanchez, David Stahl, Sue
Stern, Greg Styduhar, and Steve Tapia.
In addition to the Rocky Mountain Research Station, financial support was
provided by Littleton Public Schools, the University of Colorado, the Department of
Environmental, Population, and Organismic Biology, the Colorado Mountain Club
Foundation, Colorado Natural History Small Grants, and the Cooper Ornithological
Society.
The support I have received from my friends and family have been profoundly
important. I thank Audrey and Jim Benedict, and Tim and Michelle Connelly, whose
friendship and encouragement have been sources of strength for over two decades. I
particularly thank my mom and dad, whose love of nature was instilled in me at a
very young age. It is to them that I owe my curiosity of life, and for that I will always
be deeply appreciative. Finally, I am especially grateful for my wife, Marlene, who
provided tremendous love and support through all years of this project when I was
spending innumerable hours at night running over mountains in search of singing
owls and hooting at the moon. She truly has been the wind in my sails and I owe the
completion of this effort to her.
viii
Table of Contents
CHAPTER
I. BACKGROUND AND OUTLINE OF DISSERTATION TOPICS
Introduction………………………………………………………… 1
Outline of dissertation topics………………………………………. 4
Literature Cited ……………………………………………………. 7
II. DEMOGRAPHY OF FLAMMULATED OWLS
Abstract …………………………………………………………… 11
Introduction ………………………………………………………. 13
Methods …………………………………………………………… 14
Results ……………………………………………………………. 21
Discussion ………………………………………………………… 33
Literature Cited …………………………………………………… 38
III. LIFETIME REPRODUCTION OF FLAMMULATED OWLS
Abstract …………………………………………………………… 43
Introduction ……………………………………………………….. 44
Methods …………………………………………………………… 46
Results …………………………………………………………….. 49
Discussion …………………………………………………………. 61
Literature Cited ……………………………………………………. 67
ix
IV. MATE AND SITE FIDELITY AND DISPERSAL IN
FLAMMULATED OWLS
Abstract …………………………………………………………… 70
Introduction ……………………………………………………….. 72
Methods …………………………………………………………… 74
Results …………………………………………………………….. 81
Discussion …………………………………………………………. 89
Literature Cited …………………………………………………… 98
V. DETERMINING HABITAT QUALITY FROM LONGTERM
DEMOGRAPHICS IN FLAMMULATED OWLS
Abstract …………………………………………………………… 105
Introduction ……………………………………………………….. 108
Methods …………………………………………………………… 112
Results …………………………………………………………….. 128
Discussion ………………………………………………………… 159
Literature Cited ……………………………………………………. 170
VI. CONCLUSIONS ………………………………………………….. 180
Literature Cited ……………………………………………………. 186
VII. LITERATURE CITED ……………………………………………. 189
x
List of Tables
Table 1 Mate improvement hypothesis; comparison of lifetime
production of owlets by original males on territories from
which females dispersed to lifetime production of owlets
by new males on territories where females dispersed.
………. 88
Table 2 Territory improvement hypothesis; comparison of total
owlets produced over 19 yr on territories where females
nested before dispersal (“original territory”) to total
owlets produced on territories where females dispersed
(“new territory”).
………. 90
Table 3 Disperser-enhancement hypothesis; comparison of mean
brood size for females before they dispersed to mean
brood size on territories after they dispersed.
………. 91
Table 4 Forest overstory and understory variables used in
correlations with demographic variables.
………. 120
Table 5 Demography on owl territories from 1981-1999. ………. 131
Table 6 Comparisons of owlets over 19 yr on territories where ……….. 138
xi
females nested before dispersal (“original territory”) to
owlets produced on territories to which those same
females dispersed (“new territory”).
Table 7 Area and percentage of forest types in owl territories,
non-territory habitat, and over the entire study area.
………. 139
Table 8 Comparison of forest structure variables among forest
types.
………. 141
Table 9 Correlations among demographic variables and forest
types within owl territories.
………. 147
Table 10 Correlations among demographic and forest structure
variables within owl territories.
………. 148
Table 11 Comparison of forest structure variables in territory vs
non-territory habitat.
………. 151
Table 12 Comparison of forest structure variables among non-
territory and three classes of territory habitat.
………. 153
Table 13 Density of cavity trees among owl territories and in non- ………. 156
xii
territory habitat.
Table 14 Comparison of relative arthropod abundance between a
high-productivity territory (A4) and a low-productivity
territory (A18) during 1998-1999.
………. 158
xiii
List of Figures
Figure 1 Location of owl territories on the Manitou
Experimental Forest study area, 1981-1999.
………. 16
Figure 2 Number of territories occupied annually by bachelor
(unpaired) males and breeding pairs, 1981-1999.
………. 23
Figure 3 Number of nesting attempts and successful nests per
yr on 14 territories, 1981-1999.
………. 25
Figure 4 Annual mean number of eggs per clutch, number of
owlets per brood, and number of owlets per successful
brood, 1981-1999.
……….. 26
Figure 5 Total eggs and fledglings produced in all territories
annually from 1981-1999.
………. 28
Figure 6 Total years of return to the study area by male and
female owls.
………. 31
Figure 7 Estimated survival curves for male and female owls. ………. 32
xiv
Figure 8 Lifetime number of nesting attempts by female and
male owls.
………. 51
Figure 9 Lifetime number of successful nests by female and
male owls.
………. 52
Figure 10 Lifetime percentage of successful nests by female and
male owls.
………. 53
Figure 11 Lifetime production of eggs by female and male owls. ………. 55
Figure 12 Lifetime production of fledglings by female and male
owls.
………. 56
Figure 13 Relationship between total breeding years and total
fledglings for female and male owls.
………. 58
Figure 14 Relationship between total eggs and fledglings for
female and male owls.
………. 59
Figure 15 Percent of total fledglings produced by varying
percentages of female and male owls.
………. 60
xv
Figure 16 Lifetime number of new mates for female and male
owls.
………. 62
Figure 17 Location of owl territories on the Manitou
Experimental Forest study area, 1981-1999.
………. 82
Figure 18 Location of owl territories on the Manitou
Experimental Forest study area, 1981-1999.
………. 115
Figure 19 Contribution of individual territories to total owlets
produced by combined territories from 1981-1999.
………. 136
Figure 20 Distribution of owl territories, cavity trees, and forest
types on the Manitou Experimental Forest study area.
………. 140
Figure 21 Correlations between demographic variables and
forest types across owl territories.
………. 146
CHAPTER I
BACKGROUND AND OUTLINE OF DISSERTATION TOPICS
Life histories of avian species have evolved to maximize lifetime reproductive
output in particular environments (Stearns 1992). Life histories, which consist of
coevolved characteristics including reproductive rate, natal and breeding dispersal,
and breeding life span, vary markedly across avian species (Moreau 1944, Lack 1954,
Ricklefs 2000). Despite variance in life histories, studies of demography have
revealed several internally-consistent patterns such as correlation between body mass,
fecundity and longevity. Large species, including many raptors, are generally long-
lived and have low fecundity while small species are typically short-lived and have
high fecundity (Newton 1979, Johnsgard 1988, Gill 1995). Large, long-lived species
also generally have low rates of annual turnover with more overlap among
generations compared to small, short-lived species (Newton 1998, Ricklefs 2000).
Investigations of life histories are important in population ecology for several
reasons. First, life history characteristics such as breeding density, reproduction, and
survival determine population dynamics. Studies of these characteristics reveal the
relative stability of populations in time and space (Grant 1986, Woolfendon and
Fitzpatrick 1984), and provide insight in determining how populations are affected by
ecological factors such as habitat quality (Newton and Marquiss 1982, Forsman et al.
1996), prey abundance (Nol and Smith 1987, Korpimaki 1988), nest predation
2
(Martin 1988, Bosque and Bosque 1995), and extreme weather (Grant 1986, Owen
and Black 1989). Life history characteristics are also important in developing avian
conservation strategies because species may respond differently to environmental
perturbations based on patterns in their reproduction and survival (Newton 1995,
Forsman et al. 1996).
Second, life history investigations that focus on lifetime reproductive success
(LRS), the total offspring raised by individuals over their lifetimes, are useful for
determining reproductive strategies of species and variance in productivity among
individuals. For example, LRS data show that Kingfishers (Alcedo atthis) exhibit a
limited but highly productive breeding life, breeding annually for no more than 4 yr,
and producing more than 20 young per yr (Bunzel and Druke 1989). In contrast, LRS
data show that Barnacle Geese (Branta leucopsis) have a long but relatively
unproductive breeding life. These waterfowl initiate breeding at nearly 7 yr, produce
fewer than 5 young per yr, and total number of offspring rarely exceeds 10 in a
lifetime that may be greater than 20 yr (Owen and Black 1989). In both species, less
than one-third of the breeding population produces 50% of all offspring (Bunzel and
Druke 1989, Owen and Black 1989). LRS also provides one of the best estimates of
individual fitness, and may allow identification of individual attributes and
environmental correlates that contribute most importantly to fitness (Newton 1989).
Third, studies of mate and site fidelity (e.g., Greenwood 1980, Gavin and
Bollinger 1988), and natal and adult dispersal (e.g., Greenwood and Harvey 1982,
Forero et al. 1999), are important because movement behaviors strongly influence
population structure and gene flow (Johnson and Gaines 1990). Moreover,
3
understanding the ecological correlates of dispersal is useful because factors such as
nesting success (Haas 1998), mate quality (Goodburn 1991), and territory quality
(Korpimaki 1988) influence dispersal rate and distance, which are vital considerations
in the development of avian conservation plans (Forsman et al. 1996).
Finally, investigations of life history characteristics figure prominently in the
reliable determination of habitat quality. Most studies have inferred the quality of
avian breeding habitats based solely on relative abundance or density (e.g., Whitmore
1977, James and Warner 1982, Wenny et al. 1993), but this approach is not always
reliable and may misrepresent suitability of habitats for breeding (Van Horne 1983,
Maurer 1986, Martin 1992). While theoretical literature strongly advocates use of
direct measures of fitness (i.e., reproduction and survival) in identifying important
breeding habitats (e.g., Van Horne 1983, Martin 1992), relatively few studies of
habitat quality have been based on demography because these data are time and
energy-intensive.
Discerning patterns in life-history characteristics is best determined from
longitudinal studies, where data are collected over entire lifetimes of marked
individuals, rather than from cross-sectional studies, where data are collected at
specific points in time and from different and unknown individuals (Newton 1989).
Longtitudinal data are necessary to distinguish effects caused by individual attributes
such as age and condition (Coulson 1966, Nol and Smith 1987, Ens et al. 1992), and
stochastic environmental variation such as extreme weather phenomena (Grant 1986,
Van Horne et al. 1997).
The overall objectives of my long-term (1981-1999) investigation on
4
Flammulated Owls (Otus flammeolus) were to determine the life history
characteristics and the components of habitat quality for a population in central
Colorado. The owl breeds in montane forests from the Rocky Mountains to the
Pacific Coast, and from southern British Columbia to Vera cruz, Mexico (McCallum
1994, Linkhart et al. 1998). Determination of the life history of Flammulated Owls is
interesting because, while the owl’s life history might be expected to be similar to
that of other raptors (e.g., low fecundity and high survival), it is one of the smallest
North American owls and annually migrates the greatest distance between summer
and winter grounds (Johnsgard 1988). The owl’s population dynamics are poorly
known anywhere in their range. These data are sorely needed because the owls are
listed as sensitive and vulnerable by the United States (USDA Forest Service; Verner
1994) and Canada (van Woudenberg 1992), primarily because they are obligate
cavity nesters and because densities have declined following timber harvests
(Marshall 1957, 1988, Phillips et al. 1964, Franzreb and Ohmart 1978). While the
owl’s habitat has been described in mostly anecdotal reports (e.g., Phillips et al. 1964,
Bull and Anderson 1978; but see Linkhart and Reynolds 1997, Linkhart et al. 1998), a
clear understanding of its habitat requirements necessitates more intensive study.
In Chapter 2, my objective was to evaluate the demography of Flammulated
Owls over the 19 yr study. Specifically, I determined the density of breeding and
bachelor (unpaired) males, reproductive parameters such as productivity and nesting
success, longevity, and survival of owls associated with 14 owl territories on 511 ha.
On the basis of these data, I also assessed the owl’s life history strategy. Compared to
5
other, larger raptors, I predicted that Flammulated Owls would show more annual
variation in density, higher annual recruitment, and shorter longevity.
In Chapter 3, my objective was to determine LRS of male and female adult
owls over 19 yr. LRS is little studied in long-lived birds including raptors, and sexual
differences in LRS have only been assessed for three species. Specifically, I
compared LRS within and between sexes, investigated life-history attributes that may
have important influences on LRS, and compared the reproductive strategy of
Flammulated Owls to other raptors. Compared to other, larger raptors, I predicted
that Flammulated Owls would have a shorter reproductive lifespan, and produce more
offspring over their lifetimes.
In Chapter 4, my objective was to quantify territory and mate fidelity and
dispersal of owls. Despite their importance in affecting population dynamics and in
developing conservation plans, these life history characteristics are poorly understood
in birds because study generally requires long time periods and large areas of
investigation (Paradis et al. 1998, Forero et al. 1999). Specifically, I describe: (1)
sex differences in return rates to the study area; (2) sex differences in territory
fidelity, and I evaluate effects of previous nest failure, breeding status (paired vs
unpaired), and return of previous mate on fidelity; (3) patterns in mate fidelity and
apparent benefits of maintaining pair bonds; and (4) sex differences in breeding
dispersal. I also tested six possible correlates of breeding dispersal.
Finally, in Chapter 5 my objective was to identify characteristics of high-
quality breeding habitats, by correlating habitat characteristics on territories with
long-term demographic parameters. Specifically, I: (1) describe demographic
6
performance (territory occupancy, reproductive success, territory tenure, pair
duration, and breeding dispersal) on territories, and identify variables that
distinguished among territories; and (2) identify components of habitat quality by
correlating habitat variables with demographic performance on territories. I
evaluated habitat quality based on two questions: (a) Across territories, was
demographic performance associated with forest type and structure (e.g., tree density,
basal area, and crown volume)? (b) Does forest structure differ among territories and
between territory and non-territory (i.e., unoccupied) habitat? I predicted that
reproductive success could be positively associated with area in ponderosa
pine/Douglas-fir, a forest type and structure with which Flammulated Owls have been
associated in other studies (McCallum 1994). I also evaluated three possible limiting
factors associated with the owl’s habitat relationships, and predicted that highest-
quality territories were characterized by highest densities of cavity trees, lowest rates
of nest predation, and greatest prey abundance.
7
LITERATURE CITED
Bosque, C., and M. T. Bosque. 1995. Nest predation as a selective factor in the evolution of developmental rates in altricial birds. Amer. Natural. 145:234-260.
Bull, E. L., and E. G. Anderson. 1978. Notes on Flammulated Owls in northeastern
Oregon. Murrelet 59:26-27. Bunzel, M. and J. Druke. 1989. Kingfisher. Pages 107-117 in Lifetime reproduction
in birds (I. Newton, Ed.). Academic Press, San Diego, California. Coulson, J. C. 1966. The influence of the pair-bond and age on the breeding biology
of the kittiwake gull Rissa tridactyla. J. Anim. Ecol. 35:269-279. Ens, B. J., M Kersten, A Brenninkmeijer, and J. B. Hulscher. 1992. Territory
quality, parental effort and reproductive success of Oystercatchers (Haimatopus ostralegus). J. Anim. Ecol. 61:703-715.
Forero, M. G., J. A. Donozar, J. Blas, and F. Hiraldo. 1999. Causes and
consequences of territory change and breeding dispersal distance in the Black Kite. Ecol. 80: 1298-1310.
Forsman, E.D., S. Destefano, M. G. Raphael, and R. J. Gutierrez. 1996.
Demography of the Northern Spotted Owl. Studies in Avian Biol. No. 17, Cooper Ornithol. Soc.
Franzreb, K. E., and R. D. Ohmart. 1978. The effects of timber harvesting on
breeding birds in a mixed-coniferous forest. Condor 80:431-441. Gavin, T. A., and E. K. Bollinger. 1988. Reproductive correlates of breeding-site
fidelity in Bobolinks (Dolichonyx oryzivorus). Ecol. 69:96-103. Gill, F. B. 1995. Ornithology. W. H. Freeman and Co., New York. 2nd Ed. 766 pp. Goodburn, S. F. 1991. Territory quality or bird quality? Factors determining
breeding success in the Magpie Pica pica. Ibis 133:85-90. Grant, P. R. 1986. Ecology and evolution of Darwin’s Finches. Princeton Univ.
Press, Princeton, New Jersey. Greenwood, P. J. 1980. Mating systems, philopatry, and dispersal in birds and
mammals. Anim. Behav. 28:1140-1162. Greenwood, P. J., and P. H. Harvey. 1982. The natal and breeding dispersal of birds.
Ann. Rev. of Ecol. and Syst. 13:1-21.
8
Haas, C. 1998. Effects of prior nesting success on site fidelity and breeding
dispersal: an experimental approach. Auk 115:929-936. James, F. C., and N. O. Warner. 1982. Relationships between temperate forest bird
communities and vegetation structure. Ecol. 63:159-171. Johnson, M. L., and M. S. Gaines. 1990. Evolution of dispersal: theoretical models
and empirical test using birds and mammals. Ann. Rev. Ecol. Syst. 21:449-480.
Johnsgard, P. A. 1988. North American owls: Biology and natural history.
Smithsonian Institution Press, Washington, D.C. Korpimaki, E. 1988. Effects of territory quality on occupancy, breeding performance
and breding dispersal in Tengmalm’s Owl. J. Anim. Ecol. 57:97-108. Lack, D. 1954. The natural regulation of animal numbers. Oxford, University Press. Linkhart, B. D. and R. T. Reynolds. 1997. Territories of Flammulated Owls: Is
occupancy a measure of habitat quality? Pp. 250-254 in Biology and conservation of owls of the northern hemisphere (J. R. Duncan, D. H. Johnson, and T. H. Nichols, Eds.). USDA Forest Serv. Gen Tech. Rep. NC-190.
Linkhart, B. D., R. T. Reynolds, and R. A. Ryder. 1998. Home range and habitat of
breeding Flammulated Owls in Colorado. Wilson Bull.. 110:342-351. Marshall, J. T., Jr. 1957. Birds of pine-oak woodland in southern Arizona and
adjacent Mexico. Pacif. Coast Avif. 32:1-125. Marshall, J. T., Jr. 1988. Birds lost from a giant sequoia forest during fifty years.
Condor 90:359-372. Martin, T. E. 1988. Processes organizing open-nesting bird assemblages:
competition or nest predation. Evol. Ecol. 2:37-50. Martin, T. E. 1992. Breeding productivity considerations: what are the appropriate
habitat features for management? Pages 455-473 in Ecology and conservation of Neoptropical migrant landbirds (J. M. Hagan III and D. W. Johnston, eds.). Smithsonian Institution Press, Washington, D.C.
Maurer, B. A. 1986. Predicting habitat quality for grassland birds using density-
habitat correlation. J. Wildl. Manage. 50:556-566.
9
McCallum, A. 1994. Flammulated Owl (Otus flammeolus). In The birds of North America, No. 93 (A. Poole and F. Gill, eds.). Academy of Natural Sciences of Philadelphia; American Ornithologists’ Union, Washington, D.C.
Moreau, R. E. 1944. Clutch size: a comparative study, with references to African
birds. Ibis 86:286-347. Newton, I. 1979. Population ecology of raptors. T & A D Poyser Ltd., London. 399
pp. Newton, I. 1989. Lifetime reproduction in birds. Academic Press, San Diego,
California. Newton, I. 1995. The contribution of recent research on birds to ecological
understanding. J. Anim. Ecol. 64:675-696. Newton, I. 1998. Population limitation in birds. Academic Press, N.Y. 597 pp. Newton, I., and M. Marquiss. 1982. Fidelity to breeding area and mate in
Sparrowhawks Accipiter nisus. J. Anim. Ecol. 51:327-341. Nol, E., and J. N. M. Smith. 1987. Effects of age and breeding experience on
seasonal reproductive success in the Song Sparrow. J. Anim. Ecol. 56:301-313.
Owen, M., and J. M. Black. 1989. Barnacle Goose. Pages 349-362 in Lifetime
reproduction in birds (I. Newton, Ed.). Academic Press, San Diego, CA. Paradis, E., S. R. Baillie, W. J. Sutherland, and R. D. Gregory. 1998. Patterns of
natal and breeding dispersal in birds. J. Anim. Ecol. 67:518-536. Phillips, A. R., J. T. Marshall, and G. Monson. 1964. The birds of Arizona. Univ.
Ariz. Press, Tucson. 212 pp. Ricklefs, R. E. 2000. Density dependence, evolutionary optimization, and
diversification of avian life histories. Condor 102:9-22. Stearns, S. C. 1992. The evolution of life histories. Oxford Univ. Press, New York.
249 pp. Van Horne, B. 1983. Density as a misleading indicator of habitat quality. J. Wildl.
Manage. 47:893-901. Van Horne, B., G. S. Olson, R. L. Schooley, J. G. Corn, and K. P. Burnham. 1997.
Effects of drought and prolonged winter on Townsend’s Ground Squirrel demography in shrubsteppe habitats. Ecol. Monogr. 67:295-315.
10
Verner, J. 1994. Current management situation: Flammulated Owls. Pages 10-13 in
Flammulated, Boreal, and Great Gray Owls in the United States: a technical conservation assessment (G. D. Hayward and J. Verner, eds.). USDA Forest Serv. Gen. Tech. Rep. RM-253.
Wenny, D. G., R. L. Claswon, J. Faaborg, and S. L. Sheriff. 1993. Population
density, habitat selection, and minimum area requirements of three forest-interior Warblers in central Missouri. Condor 95:968-979.
Whitmore, R. C. 1977. Habitat partitioning in a community of passerine birds.
Wilson Bull. 89:253-265. Woolfenden, G. E., and J. W. Fitzpatrick. 1984. The Florida Scrub Jay:
demography of a cooperative-breeding bird. Monogr. Pop. Biol. 20, Princeton Univ. Press, Princeton, NJ.
van Woudenberg, A. M. 1992. Integrated management of Flammulated Owl
breeding habitat and timber harvest in British Columbia. Masters thesis, Univ. British Columbia, Vancouver.
11
CHAPTER II
DEMOGRAPHY OF FLAMMULATED OWLS IN COLORADO
Abstract. - I investigated the demography of Flammulated Owls (Otus flammeolus) in
Colorado from 1981-1999. Fourteen territories occurred on the 511 ha study area
during the 19 yr study, most of which were constant in time and space despite
periodic turnover of individuals on territories. Mean (+ SE) annual density was 0.9 +
0.1 breeding pairs 100 ha-1 and 0.7 + 0.1 bachelor (unpaired) males 100 ha-1.
Numbers of territories occupied by breeding pairs annually was relatively constant,
totaling 4 – 5 pairs in 79% of yr. Nesting success was high; 82% of 79 nests
successfully fledged > 1 owlet. Most (13 of 14) nesting failures occurred during
incubation due to predation, and failures appeared to be caused by pine squirrel
(Tamiasciurus hudsonicus) predation. Productivity of pairs varied little during the
study. Mean clutch size was 2.5 + 0.1 eggs (range = 1-3, n = 29) and mean brood size
of successful nests was 2.4 + 0.1 owlets (range = 1-4, n = 68). Total annual
production of all pairs on the study area was 9.6 + 0.5 eggs and 8.2 + 0.5 fledglings.
Reproductive success was not correlated with breeding experience; dates of initiation
of incubation and brood size were not different for either males or females breeding
for the first time on the study area vs those that bred > 2 yr. Total number of yr
banded males returned to the study area was significantly greater than for females
(3.2 + 0.6 yr vs 2.0 + 0.3 yr). Annual return rate was greater for males (0.59; 17 of
12
29) than for females (0.37; 14 of 38); either adult males had higher survival rates than
females or females more often dispersed from the study area. Reproduction and
survival indicate that Flammulated Owls have a life history strategy more typical of
larger birds, and relatively constant densities of breeding pairs suggest the owls bred
in a stable environment.
13
INTRODUCTION
Birds have diverse life histories, resulting from the cumulative evolutionary
effect of many abiotic and biotic factors whose relative importance is still debated
(Moreau 1944, Lack 1954, Ricklefs 2000). Despite wide variance among species,
studies of demography have revealed several patterns in life histories. Large species,
including many raptors, are generally long-lived and have low fecundity while small
species are typically short-lived and have high fecundity (Newton 1979, Johnsgard
1988, Ricklefs 2000). In addition, mortality rates generally decline with age, with
adults surviving at higher rates than juveniles (e.g., Perrins and Geer 1980,
Woolfenden and Fitzpatrick 1984), while nesting success and annual number of
young produced generally increases with age (e.g., Curio 1983, Nol and Smith 1987).
Life histories have strong influence on avian population dynamics. Large,
long-lived species, such as Northern Goshawks (Accipiter gentilis; Reynolds et al., in
prep.) and Northern Spotted Owls (Strix occidentalis; Forsman et al. 1996), tend to
have relatively small fluctuations in annual population densities in response to short-
term environmental oscillations because overlapping generations result in low
turnover rates by individuals on territories. Populations of these species also contain
a relatively large non-breeding segment, typically consisting of immature individuals,
which buffer the population against environmental fluctuations (Forsman et al. 1996).
In contrast, small, short-lived species, such as the Blue Tit (Parus caeruleus; Dhondt
1989), show high turnover rates of individuals on territories due to little overlap in
generations, resulting in large annual fluctuations in population densities. However,
because of high fecundity, populations of small, short-lived species can rebound more
14
quickly than large, long-lived species (Newton 1998).
I studied the demography of a Flammulated Owl population (Otus
flammeolus) in Colorado from 1981-1999 using capture-recapture methods. This
insectivorous owl is a Neotropical migrant that breeds in montane forests of western
North America as far north as southern British Columbia and winters as far south as
El Salvador (McCallum 1994, Linkhart et al. 1998). The owl’s population dynamics
are poorly known anywhere in its range. These data are sorely needed because the
owl is listed as sensitive and vulnerable by the United States (USDA Forest Service;
Verner 1994) and Canada (van Woudenberg 1992), primarily because they are
obligate cavity nesters and because densities have declined following timber harvests
(Marshall 1957, 1988, Phillips et al. 1964, Franzreb and Ohmart 1978).
Here I report data on the density, reproduction, longevity, and survival of
adult Flammulated Owls. I predicted that Flammulated Owls, which are the second-
smallest North American Owl (Johnsgard 1988), should have higher annual variance
in density, higher annual recruitment, and shorter longevity than other, larger raptors.
Elsewhere I report on other life history characteristics, including lifetime
reproduction (Chapter 3), territory and mate fidelity and dispersal (Chapter 4), and the
determination of habitat quality based on long-term demography (Chapter 5).
METHODS Study Area
The study was conducted on the Manitou Experimental Forest in Teller Co.,
Colorado. I established boundaries of the 511 ha study area after initial surveys
(1980) for territorial Flammulated Owls. After I confirmed the presence of owls,
boundaries were drawn around a sufficient area to contain approximately 20
15
territorial males, based on an estimate of territory size for this species (274 m in
diameter; Marshall 1939). Forests within the study area consist of (1) ponderosa pine
(Pinus ponderosa) mixed with Douglas-fir (Pseudotsuga menziesii), generally on
ridgetops and south- and west-facing slopes, (2) quaking aspen (Populus tremuloides)
stands on lower slopes and bottoms of moist drainages, (3) quaking aspen stands
mixed with blue spruce (Picea pungens) in bottoms, lower slopes, and benches in
mesic areas, and (4) Douglas-fir mixed with blue spruce on higher slopes and on
north-facing slopes. The area has been characterized by an absence of management
activities since the 1880’s, when there was a single-tree selection cut for railroad ties
(Reynolds et al. 1985). Snags and trees with cavities are relatively abundant
throughout the study area (Reynolds et al. 1985). The forest understory, consisting of
over 100 species of grasses, forbs, and shrubs, is poorly developed in all but the moist
creek bottoms (Reynolds et al. 1985). Terrain is moderately steep (20-80% slope)
and elevations ranged from 2,550-2,855 m.
Territory Occupancy and Population Density
Each spring and summer from 1981-1999, I systematically searched the entire
study area for territorial males (Reynolds and Linkhart 1984). I identified territory
boundaries by marking territorial song-trees of males (Reynolds and Linkhart 1984)
and by monitoring movements with radio-telemetry (1982-1983) which assisted in
determining range in territory sizes and identifying topographic characteristics of
boundaries (Linkhart et al. 1998). Boundaries of territories changed little from year
to year (Fig. 1), despite turnover of males on territories (Chapter 5). Replacement
males often sang from the same tree or groups of trees throughout their territories and
16
Figure 1. Location of owl territories (black polygons) on the Manitou Experimental
Forest study area (white boundary), 1981-1999. Heavy white polygons
represent territories (A15 and A24) not occupied after 1984 and light white
polygons represent territories (A4, A8, A11, and A29) prior to boundary shifts.
17
along boundaries that adjoined neighboring territories (Linkhart 1984, Linkhart et al.
1998). I may have missed some annual fluctuation of territory boundaries, especially
where there were no adjacent neighbors or when adjacent territories were not
occupied. However, lack of change in most boundaries indicated that territories were
relatively constant in time and space.
I mapped all suitable nest cavities (tree cavities with entrance diameters > 4
cm) within territories and checked each for nesting owls each year (Reynolds and
Linkhart 1984). Singing by nesting males dramatically declined after egg-hatch,
whereas bachelor (unpaired) males typically sang throughout a breeding season
(Reynolds and Linkhart 1987). Due to seasonal nest searching efforts, I was
confident that all nests were located each year. I determined annual density of
breeding pairs and bachelor males each yr by converting each to the frequency per
100 ha.
Nest Success and Reproduction
I found most nests during incubation (late May and early June) and I checked
the status of nests at least weekly (often two or three times per week) until the young
fledged (mid July). Breeding adults were captured at nests (occasionally on day
roosts) and banded with U. S. Fish and Wildlife Service leg bands (Reynolds and
Linkhart 1984). Sex of adults was determined by behavior at nests and weight (see
Reynolds and Linkhart 1984). I banded owlets at 14-24 days after hatching (fledging
occurs at 22-24 days; Reynolds and Linkhart 1987). A total of 215 Flammulated
Owls were banded on the study area from 1981-1999: 146 owlets (sex not
18
determined) and 69 adults (29 males and 40 females). Because bachelor males were
difficult to capture, all banded males but one were owls that bred at least once.
I defined a nesting attempt as a nest in which > 1 egg was laid, and a
successful nest as a nest that fledged > 1 owlet. I determined annual nesting success
by dividing total number of successful nests in a year by total nesting attempts in that
year. Total nesting success was the total number of successful nests over all years
divided by total nest attempts over all years. Number of eggs per clutch was
determined during egg-laying or incubation, and number of owlets per brood was
determined at the time owlets were banded. To determine total annual production of
eggs and owlets, each year I summed eggs and owlets from all clutches and broods.
For calculating nesting success, mean clutch and brood sizes, and mean annual
production of eggs and owlets, I excluded 6 nests whose outcome was uncertain (two
in 1981 and one in 1990) or failed due to human disturbance (one each in 1984, 1991,
and 1994).
To determine if breeding experience influenced the timing of breeding or
number of young produced, I compared mean dates of initiation of incubation and
mean owlets per brood for males and females breeding for the first time on the study
area vs males and females that bred two or more years on the study area. I could not
determine if males or females bred previously before their tenure on the study area.
Initiation of incubation was determined by observations of females incubating eggs,
and by back-dating from hatching (mean duration of incubation was 22 d; Reynolds
and Linkhart 1987) for nests discovered after females initiated incubation.
Return Rate, Longevity, and Survival
19
Annual return rate to the study area, longevity, and survival were determined
from capture-recapture of adults banded from 1981-1999. I determined sex-specific
return rates to the study area by dividing the number of banded males and females
that were recaptured for > 2 consecutive yr by total number of banded owls
(excluding one male and two females banded in 1999). Longevity was conservatively
estimated by summing total yr of return to the study area. I assumed that individuals
died if they failed to return to the study area. Some individuals, especially females,
may have dispersed from the study area rather than died, since females had higher
rates of breeding dispersal (0.22) on the study area than males (0.02; Chapter 4).
To estimate adult survival, I divided the number of adults captured or
recaptured in yr X that returned to breed in yr X + 1 by the total number of adults
captured or recaptured in yr X. I also plotted an approximate survival curve for each
sex following Murphy (1996), in which I treated the year an owl was banded as yr 0
and converted the number of adults banded each yr to an initial population of 1000. I
then averaged the number of owls in each cohort (1981-1998) that returned in the first
yr after banding (= yr 1) to determine survival over the first yr of banding. To
determine survival from yr 1 to yr 2, I excluded those owls banded in 1998 because
they did not yet have a second yr of potential survival, and averaged the number of
owls alive for each cohort banded in 1981-1997. For each subsequent yr, I followed
the same procedure for estimating survival. This procedure potentially
underestimates survival because it does not account for recapture efficiency, the
probability of recapturing individuals (Martin et al. 1995). Because my sample sizes
were insufficient for calculating recapture probability, I could not use capture-
20
recapture models of survival such as those used in program MARK (Lebreton et al.
1992).
When calculating return rate, longevity, and survival, I assumed that a male
banded on a territory in yr X was present on the same territory in yr X + 1, despite
being uncaptured in yr X + 1, if: (1) a male was heard singing on the territory in yr
X + 1, and (2) the same male was recaptured in a subsequent yr on the same territory.
This assumption was supported by the fact that I noted no instances of males leaving
a territory for > 1 yr and returning in a subsequent yr. No unpaired females were
detected on territories in any yr, and I detected no instances in which a female failed
to nest in one yr but nested on the same territory in the previous yr and subsequent yr.
Therefore, I presumed a banded female that nested in yr X on a territory had not
returned to the same territory in yr X + 1 if no nest was found. Because I could not
distinguish between mortality and dispersal away from the study area, estimates of
longevity and survivorship are conservative.
Estimates of longevity and survival for adults banded in 1981 with unknown
histories and adults banded and/or recaptured in 1999 with unknown futures (‘fringe’
adults) did not significantly differ from adults whose breeding lifespans began after
1981 and terminated before 1999 (‘inclusive’ adults). Mean (+ SE) total yr of return
for ‘fringe’ males (n = 4 in 1981 and 5 in 1999) was 3.9 + 1.3 yr, and mean total yr of
return for ‘inclusive’ males was 2.6 + 0.5 yr (n = 22; t = -0.94, df = 11, P = 0.37, two-
tailed). Likewise, mean total yr of return for ‘fringe’ females (n = 5 in 1981 and 5 in
1999) was 2.4 + 0.6 yr and mean total yr of return for ‘inclusive’ females was 1.7 +
0.3 yr (n = 31; t = -1.03, df = 14, P = 0.32, two-sided). Consequently, to maximize
21
sample sizes I included the 9 adults banded in 1981 and the 10 adults banded and/or
recaptured in 1999 for calculating estimates of longevity and survival.
Statistical analyses were performed using Statistical Analysis System (SAS
Institute 1995). I used unpaired t-tests (two-tailed) assuming unequal variances
(PROC TTEST) to determine effect of breeding experience on mean date of initiation
of incubation and mean brood size. I used Wilcoxon’s test (PROC NPAR1WAY) to
evaluate sex differences in longevity, and Fisher’s exact test (PROC FREQ) to
evaluate sex differences in annual return rates to the study area. Throughout the
paper I report means + SE. Analyses were considered significant if P < 0.05.
RESULTS
Territory Occupancy
Fourteen owl territories were located on the 511 ha study area between 1981
and 1999 (Fig. 1). With a few exceptions, territories were generally fixed in time and
space despite individual turnover on each territory. First, territory A24 was only
occupied from 1981-1983. In 1984, a new male in an adjacent territory (A29)
expanded his territory to include the western portion of territory A24. The new
boundaries of A29 did not change over the remainder of the study (Fig. 1). Second,
the male in territory A4 expanded his territory in 1983 to include much of the eastern
portion of territory A15, which was only occupied in 1981-1982 and 1984 (Linkhart
et al. 1998; Fig. 1). I did not determine the boundaries of either A4 or A15 in 1984,
when only A15 territory was occupied by a breeding pair, but for the remainder of the
study (1984-1999), territory A4 contained the eastern half of territory A15. Finally,
in 1988 the male in territory A8 expanded his territory north into the southern portion
22
of A11 territory when the male in territory A11 did not return from migration, and
northeast into the northern portion of territory A15 (Fig. 1). Beginning in 1989,
territory A11 contained only the northern portion of the original territory A11 (Fig.
1). In each of the above instances, shifts in territory boundaries occurred when males
in adjacent territories did not return from spring migration. Boundaries of all other
territories remained unchanged over the study. Because males had high territory
fidelity (98%; Chapter 4), newly arriving males in the spring typically filled
geographic voids left by predecessors.
Population Density
Of the total 14 owl territories on the study area, males annually occupied a
mean 8.1 + 0.5 (58 + 4%) territories, ranging from 4 territories (29%) in 1986 to 13
territories (93%) in 1995. Mean annual density of territorial males was 1.7 + 0.1 100
ha-1. However, annual density of breeding pairs was 0.9 + 0.1 100 ha-1, reflecting the
fact that breeding pairs only occupied a mean 4.5 + 0.2 territories annually (32 + 3%;
Fig. 2). Nonetheless, frequency of territories occupied by breeding pairs each yr
remained relatively constant, between 3 and 6 territories, and between 4 and 5
territories in 79% of all yr (15 of 19; Fig. 2). Annual density of bachelor males was
0.7 + 0.1 100 ha-1, reflecting the fact that bachelor males occupied a mean 3.6 + 0.5
territories annually (26 + 3%; Fig. 2). While mean density of bachelor males was
similar to mean density of breeding pairs, frequency of territories annually occupied
by bachelor males was more variable, ranging from one in 1986 to eight in 1995 (Fig.
2).
23
Figure 2. Number of territories occupied annually by bachelor (unpaired) males
(light bars) and breeding pairs (dark bars), 1981-1999.
0
1
2
3
4
5
6
7
8
9
81 83 85 87 89 91 93 95 97 99
Year
No.
terr
itorie
s ye
ar-1
24
Nest Success and Reproduction
Annual Nesting success.—I recorded 85 total nesting attempts. There were no second
nesting attempts, even when nests failed in early incubation. Thus, mean total nesting
attempts per yr (4.5 + 0.2, range 3 - 6) was identical to mean total breeding pairs per
yr. Mean total successful nests was 3.4 + 0.2 annually (n = 79), ranging from 2 in
1993 and 1994 to 5 in 1982 (Fig. 3). Nesting success was high; 82% (65 of 79; 6
nesting attempts were excluded because I was uncertain whether young fledged [3],
or they failed due to human disturbance [3]) of all nests successfully fledged young.
Nesting success was 100% in 8 yr, 75-99% in 8 yr, 50-74% in 2 yr, and < 50% in 1
yr. A total of 14 nesting failures occurred in 9 of the 14 territories. One failure was
caused by predation of the female during late incubation. The remaining failures
were caused by predation on eggs or owlets during incubation or early nestling
periods. Although predators responsible for nesting failures were unknown, pine
squirrels (Tamiasciurus hudsonicus) were the most likely predator as few other
predaceous mammals (excepting one bushytail woodrat, Neotoma cinerea; also see
Linkhart and Reynolds 1994), and no predaceous birds, were ever observed in tree
cavities.
Reproduction.—Mean number of eggs per clutch and owlets per brood varied little
over the study. Mean clutch size over all yr was 2.5 + 0.1 eggs (n = 29; Fig. 4) and
all clutches contained two or three eggs. Combining successful and unsuccessful
nests, mean brood size was 2.0 + 0.6 owlets (n = 79), ranging from 0.6 owlets in 1993
to 3.0 owlets in 1990, while only successful nests over all yr contained 2.4 + 0.1
owlets (n = 65), ranging from 1.5 owlets in 1990 to 3.0 owlets in 1990 (Fig. 4). Other
25
Figure 3. Number of nesting attempts (dark bars) and successful nests (light bars) per
yr on 14 territories, 1981-1999 (n = 79; these data exclude 3 nests whose
outcome was uncertain [two in 1981 and one in 1990] and 3 nests that failed
due to human disturbance [one each in 1984, 1991, and 1994]).
0
1
2
3
4
5
6
81 83 85 87 89 91 93 95 97 99
Year
Tot
al n
estin
g at
tem
pts
or s
ucce
ssfu
l nes
ts
26
Figure 4. Annual mean number of eggs per clutch (dark bars; n = 29), number of
owlets per brood (white bars; n = 79), and number of owlets per successful
brood (stippled bars; n = 65), 1981-1999. These data exclude 3 nests whose
outcome was uncertain (two in 1981 and one in 1990) and 3 nests that failed
due to human disturbance (one each in 1984, 1991, and 1994).
Year
Mea
n cl
utch
or
bro
od s
ize
0
0.5
1
1.5
2
2.5
3
3.5
4
81 83 85 87 89 91 93 95 97 99
27
than one brood with one owlet and one brood with four owlets, all broods contained
two or three owlets. Similarity between mean clutch size and mean brood size
reflected high survival of eggs; 79% of all eggs (57 of 72; n = 29 clutches) over the
study survived to hatching, a value which was similar to nesting success because
predation, when it occurred, always resulted in the loss of entire clutches. Survival of
owlets was even higher; 95% of owlets (144 of 151; n = 61 broods) survived from
egg-hatching to fledging. Because no nests failed after the midpoint (day 12) of the
nestling period (22-24 d; Reynolds et al. 1987), mean number of fledglings per brood
was identical to mean number of banding-age owlets per brood. Survival of
fledglings prior to fall migration was unknown, but was probably low based on the
fact that 4 of 8 (50%) fledglings in four radio-tagged broods were killed by predators
in the first six weeks after they fledged in 1982-83 (Linkhart 1984).
Total eggs and fledglings produced annually by all breeding pairs were nearly
equivalent; owls annually produced a mean total 9.6 + 0.5 eggs and 8.2 + 0.5
fledglings (Fig. 5). Range in total eggs produced on the study area annually was
attributable to variance in number of breeding pairs since range in clutch sizes was
small; the fewest total eggs (5) were produced in 1986 when there were 3 breeding
pairs and the most total eggs (13) were produced in 1999 when there were 5 breeding
pairs. Difference between total eggs and total fledglings produced annually was
attributable to nest predation; the fewest fledglings (3) were produced in 1993 when 3
of 5 nests were lost to predators and the most fledglings (11) were produced in 1982
and 1985 when all nests were successful.
Effect of Breeding Experience on Reproductive Success—In many birds, increased
28
Figure 5. Total eggs (dark bars) and fledglings (light bars) produced in all territories
annually from 1981-1999. These data exclude 3 nests whose outcome was
uncertain (two in 1981 and one in 1990) and 3 nests that failed due to human
disturbance (one each in 1984, 1991, and 1994).
Year
Tot
al e
ggs
or fl
edg
ed o
wle
ts
0
2
4
6
8
10
12
14
81 83 85 87 89 91 93 95 97 99
29
breeding experience of adults is associated with earlier egg-laying and increased sizes
of clutches and broods (Pietiainen 1988). I assessed effect of breeding experience of
male and female Flammulated Owls on date of initiation of incubation and brood size.
For males breeding for the first time on the study area, incubation began 1 d later than
males with > 2 yr breeding experience (5 June + 2 d [n = 14, range 26 May-16 June],
vs 4 June + 1 d [n = 28, range 19 May –29 June]), but this difference was not
significant (t = 0.36, df = 24, P = 0.72). Similarly, females breeding for the first time
initiated incubation 2 d later than females with > 2 yr breeding experience (5 June + 2
d [n = 21, range 27 May-29 June], vs 3 June + 2 d [n = 21, range 19 May-24 June]),
but the difference also was not significant (t = 0.94, df = 40, P = 0.35). Breeding
experience was not associated with productivity for either sex. Males breeding for
the first time on the study area had a mean brood size similar to males with > 2 yr
breeding experience (2.3 + 0.2 owlets [n = 16, range 0-3], vs 2.2 + 0.2 owlets [n = 35,
range 0-4]; t = 0.40, df = 49, P = 0.69). Likewise, females breeding for the first time
had mean brood size similar to females with > 2 yr breeding experience (2.1 + 0.2
owlets [n = 23, range 0-3], vs 2.3 + 0.2 owlets [n = 28, range 0-4]; t = -0.72, df = 49,
P = 0.47). I could not determine if male or female owls bred previously before their
tenure on the study area.
Return Rate, Longevity, and Survival
For sexes combined, rate of return to the study area for > 2 consecutive yr was
0.46 (31 of 67). Return rate was greater for males (0.59; 17 of 29) than for females
(0.37; 14 of 38), but not significantly so (Fisher’s exact test, P = 0.09). Only two
30
owls (both females) returned to breed on the study area after each was undetected for
one breeding season (one in 1998 and 1999). Longevity, estimated by total yr of
return to the study area, was 2.5 + 0.3 yr for all adult owls (includes all breeders and
one bachelor male). However, mean total yr of return was greater for males (3.2 +
0.6 yr, n = 30) than for females (2.0 + 0.3 yr, n = 41; z = 2.0, P = 0.04; Fig. 6). Forty
percent of males returned for > 3 yr to the study area compared to just 18% of
females. Maximum yr of return was 12 yr for males and 8 yr for females. Because
age could not be determined when adults were first banded, total yr of return
represent minimum estimates of longevity. I documented only one instance of natal
dispersal; a male hatched in 1981 was found breeding on another territory 1.4 km
distant from 1987-1989 (Chapter 4). Age of this male was 8 yr, 1 mo in 1989.
Annual frequency of return likely provides a good basis for estimating
survival rates for male Flammulated Owls, given their high territory fidelity (98%;
Chapter 4). At 3 yr after banding, 32% of males (n = 30) were estimated to be alive,
and at 6 yr after banding, 15% of males were estimated to be alive (Fig. 7). At 10 yr
after banding, 11% of males were estimated to be alive, although this estimate was
based on just 2 males (Fig. 7). Annual frequency of return may underestimate
survival of females compared to males, due to their higher rates of breeding dispersal
(0.22 vs 0.02 for males; Chapter 4). Compared to males, survival estimates for
females (n = 41) were lower for all yr after banding; just 12% of females were
estimated to be alive 3 yr after banding (Fig. 7).
31
Figure 6. Total years of return to the study area by male (light bars, n = 30) and
female (dark bars, n = 41) owls. All owls were banded as adults at unknown
ages.
0
20
40
60
80
100
1 2 3 4 5 6 7 8 9 10 11 12
Years
Per
cent
of t
otal
mal
es o
r fe
mal
es
32
Figure 7. Estimated survival curves for male (solid line, n = 30) and female (dashed
line, n = 41) owls. All owls (except for one male )were banded as adults at
unknown ages.
Years since first banded
No.
indi
vidu
als
surv
ivin
g
0
200
400
600
800
1000
0 1 2 3 4 5 6 7 8 9 10 11 12
33
DISCUSSION
Life History
Several demographic characteristics indicate that Flammulated Owls, which
are the second-smallest North American strigiform (Johnsgard 1988), have a life
history more typical of larger birds, which are long-lived and have low fecundity
(Newton 1998). First, annual reproduction of Flammulated Owls is among the lowest
and least variable of North American and European strigiforms. In this study, mean
clutch size was 2.5 + 0.1 eggs and mean brood size (successful and unsuccessful) was
2.0 + 0.1 owlets. Size of clutches and broods was typically 2 or 3; I only observed
one brood of 1 owlet and one brood of 4 owlets. Only two other owls, the Great
Horned Owl (Bubo virginianus; 1300-1700 g) and Barred Owl (Strix varia; 600-800
g), much larger than Flammulated Owls (50-66 g; Reynolds and Linkhart 1987), have
a smaller clutch size (-x = 2.4 eggs/clutch for both species; Murray 1976, Johnson
1978). Second, at least 75% of nests were successful in 16 of 19 years for an overall
nesting success rate of 82% over the study. Among North American strigiforms, only
Spotted Owls (S. occidentalis) have a higher nesting success (85%; Forsman et al.
1984, Johnsgard 1988). Third, I never observed replacement clutches or multiple
broods in Flammulated Owls even when nests failed early in the breeding season.
Although multiple broods are relatively uncommon among raptors (but see Marti
1997), several small and some medium-sized species in temperate regions may lay
replacement clutches if their first clutch is lost in early incubation (Newton 1979,
Johnsgard 1988). Finally, while longevity is not known for most North American
34
strigiforms, male Flammulated Owls have greater maximum longevity (12+ yr) than
other small (< 150 g) owls and a longevity comparable to many larger owls (Glutz
and Bauer 1980, Clapp et al. 1983, Klimkiewicz and Futcher 1989, Klimkiewicz,
pers. comm). Coupled with low turnover rates on territories (Chapter 5), these data
suggest that male Flammulated Owls have relatively high annual survival, despite
their being the most migratory of North American strigiforms (Johnsgard 1988).
Breeding Densities
Density of territories occupied by breeding pairs varied little over the 19 yr
study; breeding pairs occupied either 4 or 5 territories in 79% (15 of 19) of yr. The
relatively constant annual breeding density suggests that the owls in the study area
occupied a relatively stable environment (sensu Pianka 1970). Other long-term
studies of raptors including strigiforms showed stable breeding populations over time
(e.g., Gargett 1977, Korpimaki 1988).
Underlying the relative constancy in annual density of breeding-pairs was the
fact that many of the same territories were reoccupied nearly every year by breeding
pairs. As a consequence of the continuous occupancy, these territories accounted for
the majority of total productivity over the study. Elsewhere I reported that 5 of 12
territories were occupied by breeding pairs > 8 yr (not necessarily consecutive) and
accounted for nearly 70% of total owlets produced on the study area from 1981-1999,
and 3 territories, occupied by breeding pairs for > 11 yr, accounted for 50% of total
owlets produced (Chapter 5). Productivity on territories was higher on territories
with mature, open forests of ponderosa pine/Douglas-fir forests (Chapter 5),
suggesting that availability of this forest type and structure were factors associated
35
with the relative constancy in density and reproduction of Flammulated Owls on the
study area. Other studies inferred that demographic parameters were most constant in
high-quality breeding habitats (e.g., Probst and Hayes 1987, Korpimaki 1988,
Steenhof et al. 1999).
Patterns in demographic characteristics in birds are often closely related to
food abundance. Several studies have found that annual densities of raptor
populations were strongly correlated with prey density (e.g., Craighead and
Craighead 1956,White and Cade 1971, Korpimaki 1988). Reproductive parameters
such as nesting success and clutch size are closely tied to food abundance in several
species of strigiforms (Johnsgard 1988) and other raptors (Steenhof et al. 1999). The
constancy of Flammulated Owl demographic characteristics during the study suggests
that prey generally was a reliable resource. Indeed, long-term owl productivity was
not associated with prey abundance sampled over 2 yr (Chapter 5), suggesting that
prey abundance was not limiting and was not associated with habitat quality.
Sex Differences In Longevity
Greater estimated longevity for male Flammulated Owls suggests a sex bias in
survival, emigration, or both. Mean total yr of return was significantly greater for
males (3.2 + 0.6 yr) than for females (2.0 + 0.3 yr), and 40% of males returned for > 3
yr to the study area compared to just 18% of females. Annual frequency of return
may give a biased estimate of survival because (1) females had higher rates of
detected breeding dispersal within the study area than males, and (2) females had
lower return rates following nest failure than males (Chapter 4). These data suggest
that some of the females that had not returned may have dispersed from the study area
36
(Chapter 4). Nonetheless, the fact that unpaired males annually occupied 10-70% of
the 14 territories strongly suggests a shortage of females. If differences in return rates
for males and females are related to survival, the underlying reasons for these
differences are uncertain. A possible explanation is that the cost of egg production in
female Flammulated Owls, whose clutches represent more of their mass (55-60%;
approximately 30 g) than most other strigiforms (Johnsgard 1988), coupled with
energetic costs of long-distance migration prior to egg-laying, may result in higher
adult female mortality. Other studies of monogamous birds have reported or
suggested a male-biased sex ratio in adults (e.g., Breitwisch 1989 and sources therein,
Burke and Nol 1998, Gibbs and Faaborg 1990, Payne and Payne 1990).
Factors Affecting Reproductive Success
Nest predation is a primary cause of nesting mortality for many bird species
(e.g., Skutch 1949, Ricklefs 1969), and is believed to be an important factor in the
evolution of life-histories (Slagsvold 1982, Sonerud 1985, Martin 1988, Bosque and
Bosque 1995). While nest predation was responsible for reducing nesting success in
certain years (e.g., 1993 and 1999), the high rate of nesting success (82%) over 19 yr
suggests that nest predation may not be a major factor influencing reproductive
success. Elsewhere I reported that, among territories differing in long-term
productivity, predation rates at artificial nests were not significantly different,
indicating that nest predation was not associated with territory quality (Chapter 5).
Contrary to studies that found that reproductive success was correlated with
breeding experience (e.g., Nol and Smith 1987, Pietiainen 1988), male and female
Flammulated Owls with > 2 yr breeding experience did not initiate incubation earlier
37
or have larger broods than males and females breeding for the first time on the study
area. However, if owls bred elsewhere prior to their first breeding on the study area,
then any differences between inexperienced (first-time) or experienced breeders
would have been diluted. Moreover, since adults at first capture could not be aged, I
could not separate the effects of breeding age from the effects of breeding experience.
Age, rather than experience per se, was known to affect reproductive success in
several birds (e.g., Harvey et al. 1979, Curio 1983, Nol and Smith 1987).
Conservation Implications
High-quality breeding habitat for Flammulated Owls in central Colorado was
characterized as mature, open forests of ponderosa pine/Douglas-fir and owls
preferentially foraged in this forest type (Chapter 5; Linkhart et al. 1998). Elsewhere
in its range, the owl has been generally associated with mature conifer forests (see
McCallum 1994), and these forests have been subjected to extensive tree harvesting
over the past several decades. In fact, tree harvesting caused population declines in
Flammulated Owls in some areas (Marshall 1957, 1988, Phillips et al. 1964, Franzreb
and Ohmart 1978). Based on the fact that the Flammulated Owl appears to have a K-
selected life-history strategy, characterized by low rates of reproduction and high
survival (Pianka 1970), these data suggest that the owl may be vulnerable to long-
term habitat changes. K-selected species typically respond slowly to environmental
perturbations because of their low fecundity and low density (Newton 1998). In order
to understand effects of habitat changes on dynamics and long-term viability of owl
populations, researchers need to undertake comparative demographic studies of owls
across multiple forest management regimes.
38
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Breitwisch, R. 1989. Mortality patterns, sex ratios, and parental investment in
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Coulson, J. C. 1966. The influence of the pair-bond and age on the breeding biology
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Co., Pennsylvania. Curio, E. 1983. Why do young birds reproduce less well? Ibis 400-404. Dhondt, A. A. 1989. Blue Tit. Pages 15-33 in Lifetime reproduction in birds (I.
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Forsman, E. D., E. C. Meslow, and H. M. Wight. 1984. Distribution and biology of
the Spotted Owl in Oregon. Wildl. Monogr. 87:1-64. Forsman, E.D., S. Destefano, M. G. Raphael, and R. J. Gutierrez. 1996.
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Franzreb, K. E., and R. D. Ohmart. 1978. The effects of timber harvesting on
breeding birds in a mixed-coniferous forest. Condor 80:431-441. Gargett, V. 1977. A 13-year population study of the Black Eagles in the Matopos,
Rhodesia, 1964-1976. Ostrich 48: 17-27.
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Gibbs, J. P., and J. Faaborg. 1990. Estimating the viability of Ovenbird and Kentucky Warbler populations in forest fragments. Conserv. Biol. 4:193-196.
Grant, P. R. 1986. Ecology and evolution of Darwin’s Finches. Princeton Univ.
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Mitteleuropas, Vol. 9. Akademische Verlagsgesellschaft, Wiesbaden. Harvey, P. H., P. J. Greenwood, C. M. Perrings, and A. R. Martin. 1979. Breeding
success of great tits Parus major in relation to age of male and female parent. Ibis 121:186-200.
Johnsgard, P. A. 1988. North American owls: Biology and natural history.
Smithsonian Institution Press, Washington, D.C. Klimkiewicz, M. K., and A. G. Futcher. 1989. Longevity records of North American
birds. Supplement I. J. Field Ornithol. 60:469-494. Korpimaki, E. 1988. Effects of territory quality on occupancy, breeding performance
and breding dispersal in Tengmalm’s Owl. J. Anim. Ecol. 57:97-108. Lack, D. 1954. The natural regulation of animal numbers. Oxford, University Press. Lebreton, J. D., K. P. Burnham, J. Clobert, and D. R. Andersen. 1992. Modeling
survival and testing biological hypotheses using marked animals: a unified approach with case studies. Ecol. Monogr. 62:67-118.
Linkhart, B. D., R. T. Reynolds, and R. A. Ryder. 1998. Home range and habitat of
breeding Flammulated Owls in Colorado. Wilson Bull.. 110:342-351. Linkhart, B. D., and R. T. Reynolds. 1994. Peromyscus carcass in the nest of a
Flammulated Owl. J. Raptor Res. 28:43-44. Linkhart, B. D. 1984. Range, activity, and habitat use by nesting Flammulated Owls
in a Colorado ponderosa pine forest. M. S. Thesis, Colorado State Univ., Fort Collins. 45 pp.
Marshall, J. T., Jr. 1939. Territorial behavior of the Flammulated Screech Owl.
Condor 41:71-78. Marshall, J. T., Jr. 1957. Birds of pine-oak woodland in southern Arizona and
adjacent Mexico. Pacif. Coast Avif. 32:1-125. Marshall, J. T., Jr. 1988. Birds lost from a giant sequoia forest during fifty years.
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Marti, C. D. 1997. Lifetime reproductive success in Barn Owls near the limit of the
species range. Auk 114:581-592. Martin, T. E. 1988. Processes organizing open-nesting bird assemblages:
competition or nest predation. Evol. Ecol. 2:37-50. Martin, T. E., J. Clobert, and D. R. Anderson. 1995. Return rates in studies of life
history evolution: are biases large? J. Appl. Stat. 22:863-875. McCallum, A. 1994. Flammulated Owl (Otus flammeolus). In The birds of North
America, No. 93 (A. Poole and F. Gill, eds.). Academy of Natural Sciences of Philadelphia; American Ornithologists’ Union, Washington, D.C.
Moreau, R. E. 1944. Clutch size: a comparative study, with references to African
birds. Ibis 86:286-347. Murphy, M. T. 1996. Survivorship, breeding dispersal and mate fidelity in Eastern
Kingbirds. Condor 98:82-92. Murray, G. A. 1976. Geographic variation in the clutch sizes of seven owl species.
Auk 93:602-613. Newton, I. 1979. Population ecology of raptors. T & A D Poyser Ltd., London. 399
pp. Newton, I. 1998. Population limitation in birds. Academic Press, New York. 597
pp. Nol, E., and J. N. M. Smith. 1987. Effects of age and breeding experience on
seasonal reproductive success in the Song Sparrow. J. Anim. Ecol. 56:301-313.
Payne, R. B., and L. L. Payne. 1990. Survival estimates of Indigo Buntings:
comparisons of banding recoveries and local observations. Condor 92:938:946.
Perrins, C. M., and T. A. Geer. 1980. The effect of Sparrowhawks on Tit
Populations. Ardea 68:133-142. Phillips, A. R., J. T. Marshall, and G. Monson. 1964. The birds of Arizona. Univ.
Ariz. Press, Tucson. 212 pp. Pianka, E. R. 1970. On r- and K-selection. Amer. Natural. 104:592-597.
41
Pietiainen, H. 1988. Breeding season quality, age, and the effect of experience on the reproductive success of the Ural Owl (Strix uralensis). Auk 105:316-324.
Probst, J. R., and J. P. Hayes. 1987. Pairing success of Kirtland’s Warblers in
marginal vs. suitable habitat. Auk 104:234-241. Reynolds, R. T., and B. D. Linkhart. 1984. Methods and materials for capturing and
monitoring Flammulated Owls. Great Basin Natural. 44:49-51. Reynolds, R. T., B. D. Linkhart, and J. Jeanson. 1985. Characteristics of snags and
trees containing cavities in a Colorado conifer forest. USDA Forest Serv. Res. Note RM-455. 6 pp.
Reynolds, R. T., and B. D. Linkhart. 1987. The nesting biology of Flammulated
Owls in Colorado. Pages 239-248 in Biology and conservation of northern forest owls: symposium proceedings (R. W. Nero, R. J. Clark, R. J. Knapton, R. H. Hamre, eds.). USDA Forest Serv. Gen. Tech.. Rep. RM-142.
Ricklefs, R. E. 1969. An analysis of nesting mortality in birds. Smithson. Contrib.
Zool. 9:1-48. Ricklefs, R. E. 2000. Density dependence, evolutionary optimization, and
diversification of avian life histories. Condor 102:9-22. SAS Institute. 1996. SAS user’s guide: Statistics, version 6.12. SAS Institute, Inc.,
Cary, North Carolina. Skutch, A. F. 1949. Do tropical birds rear as many young as they can nourish? Ibis
91:430-455. Slagsvold, T. 1982. Clutch size variation in passerine birds: the nest predation
hypothesis. Oecologia 54:159-169. Sonerud, G. A. 1985. Risk of nest predation in three species of hole nesting owls:
influence on choice of nesting habitat and incubation behaviour. Ornis Scand. 16:261-269.
Steenhof, K., M. N. Kochert, L. B. Carpenter, and R. N. Lehman. 1999. Long-term
Prairie Falcon population changes in relation to prey abundance, weather, land uses, and habitat conditions. Condor 101:28-41.
Van Horne, B., G. S. Olson, R. L. Schooley, J. G. Corn, and K. P. Burnham. 1997.
Effects of drought and prolonged winter on Townsend’s Ground Squirrel demography in shrubsteppe habitats. Ecol. Monogr. 67:295-315.
42
van Woudenberg, A. M. 1992. Integrated management of Flammulated Owl breeding habitat and timber harvest in British Columbia. Masters thesis, Univ. British Columbia, Vancouver.
Verner, J. 1994. Current management situation: Flammulated Owls. Pages 10-13 in
Flammulated, Boreal, and Great Gray Owls in the United States: a technical conservation assessment (G. D. Hayward and J. Verner, eds.). USDA Forest Serv. Gen. Tech. Rep. RM-253.
White, C. M., and T. J. Cade. 1971. Cliff-nesting raptors and ravens along the
Colville River in arctic Alaska. Living Bird 10: 107-150. Wiens, J. A. 1986. Spatial scale and temporal variation in studies of shrubsteppe
birds. Pages 154-172 in Community ecology (J. Diamond and T Case, Eds.). Harper and Row Publishers, New York.
Woolfenden, G. E., and J. W. Fitzpatrick. 1984. The Florida Scrub Jay:
demography of a cooperative-breeding bird. Monogr. Pop. Biol. 20, Princeton Univ. Press, Princeton, NJ.
van Woudenberg, A. M. 1992. Integrated management of Flammulated Owl
breeding habitat and timber harvest in British Columbia. Masters thesis, Univ. British Columbia, Vancouver.
43
CHAPTER III
LIFETIME REPRODUCTION OF FLAMMULATED OWLS
Abstract. - I investigated lifetime reproduction of 22 male and 37 female adult
Flammulated Owls (Otus flammeolus) on 511 ha in Colorado from 1981-1999. Mean
(+ SE) clutch size over the study was 2.5 + 0.1 eggs and mean brood size for
successful nests was 2.4 + 0.1 owlets. Total years that individual owls bred
successfully (i.e., produced at least one owlet/yr) was 1.7 + 0.3 yr (range 1 to 7 yr, n
= 36) for females and 2.5 + 0.4 yr (range 1 to 9 yr, n = 22) for males. Individual
females produced a total 4.9 + 0.8 eggs (range 1 to 19 eggs) and 4.1 + 0.7 owlets
(range 0 to 18 owlets), and males produced 6.8 + 1.1 eggs (range 2 to 25 eggs) and
6.1 + 1.0 owlets (range 0 to 22 owlets), in their lifetimes. Relatively few individuals
accounted for a large proportion of the total production of offspring. Seventeen
percent of 37 females produced 50% of total owlets, while 27% of 22 males produced
50% of total owlets. Adults with longer breeding lifespans produced more owlets in
their lifetimes, and differences between sexes in lifetime reproduction and lifetime
number of mates were associated with greater longevity on the study area by males.
Flammulated Owls have breeding lifespans comparable to other, larger strigiforms,
and coupled with the fact that I never observed renesting or multiple broods, these
data support reports that the owl generally has a K-selected life-history strategy.
44
INTRODUCTION
Avian reproductive strategies have been shaped by a variety of environmental
factors including climate, habitat, food resources, and predators (Stearns 1976,
Southwood 1977). These factors have diversified reproductive strategies among
species by influencing parameters such as age of first breeding (e.g., Pietiainen 1988),
sex- and age- dependent reproduction (e.g., Pianka 1970, Pianka and Parker 1975,
Orians and Beletsky 1989, Fitzpatrick and Woolfenden 1989), and breeding lifespan
(e.g., Owen and Black 1989). Studying reproductive strategies is important because
they offer insight into evolutionary factors influencing reproductive rate (e.g., Lack
1947, Skutch 1949), and because this information helps determine how species may
respond to environmental changes (Newton 1998).
Reproductive strategies of species are perhaps best determined by
measurements of lifetime reproductive success (LRS), the total offspring raised by
individuals over their lifetimes. LRS, which is based on longitudinal studies,
provides more accurate estimates of reproductive contributions by individuals and of
variation in reproductive output among individuals than annual measures from cross-
sectional studies (Newton 1989a, Clutton-Brock 1988). LRS may also provide the
best estimate of biological fitness for individuals because these data elicit information
on the relative contribution of individual genotypes, and allow identification of traits
that may most contribute to fitness (Williams 1966, Newton 1989a).
LRS studies have revealed patterns in reproductive strategies and variance in
productivity among birds. Small species generally have shorter but more productive
45
breeding lives than large species. For example, the relatively small Kingfisher
(Alcedo atthis) breeds at less than one yr, seldom lives more than 4 yr, and may
produce more than 20 young per yr (Bunzel and Druke 1989). In contrast, the much
larger Barnacle Geese (Branta leucopsis) generally initiates breeding at 7 yr, and
produces no more than 10 offspring in a lifespan that may exceed 20 yr (Owen and
Black 1989). Despite ecological differences among birds, most studies of LRS have
shown that a few individuals produce a disproportionately large percentage of the
next generation (Clutton-Brock 1988, Newton 1989a). In both the Kingfisher and
Barnacle Geese, for example, fewer than one-third of the breeding population
produces 50% of all offspring (Bunzel and Druke 1989, Owen and Black 1989).
LRS is little studied among raptors because most of these species are
relatively long-lived. LRS has been estimated for Eurasian Sparrowhawk (Accipiter
nisus; Newton 1989b), Merlin (Falco columbarius; Wiklund 1995), Eastern Screech
Owl (Otus asio; Gehlbach 1989), Ural Owl (S. uralensis; Saurola 1989), Osprey
(Pandion heliaetus; Postupalsky 1989), Tengmalm’s Owl (Aegolius funereus;
Korpimaki 1992), and Barn Owl (Tyto alba; Marti 1997).
I present LRS data for a population of Flammulated Owls (O. flammeolus) in
Colorado studied for 19 years (1981-1999). The Flammulated Owl is a cavity-nester
that breeds in montane forests of western North America (McCallum 1994, Linkhart
et al. 1998). These owls are entirely insectivorous, feeding mostly on small moths
(Reynolds and Linkhart 1987), and are among the most migratory of all North
American strigiforms (Johnsgard 1988), breeding as far north as southern British
Columbia and wintering as far south as El Salvador (McCallum 1994).
46
Determination of LRS in Flammulated Owls is interesting because, while the owl’s
lifetime reproduction might be expected to be similar to that of other raptors (e.g.,
long breeding lifespan), it is one of the smallest North American owls and annually
migrates the greatest distance (Johnsgard 1988). Specifically, my objectives were to:
(1) describe individual variation in LRS among both sexes; (2) compare LRS
between sexes; (3) identify life-history attributes that have important influences on
LRS; and (4) compare the reproductive strategy of Flammulated Owls to other
raptors. Compared to other, larger raptors, I predicted that Flammulated Owls would
have shorter reproductive lifespans, and produce more offspring over their lifetimes.
METHODS
Study Area
The study area was a 511 ha tract on the Manitou Experimental Forest in
Teller Co., Colorado. Forests within the study area consisted of (1) ponderosa pine
(Pinus ponderosa) mixed with Douglas-fir (Pseudotsuga menziesii), generally on
ridgetops and south- and west-facing slopes, (2) quaking aspen (Populus tremuloides)
stands on lower slopes and bottoms of moist drainages, (3) quaking aspen stands
mixed with blue spruce (Picea pungens) in bottoms, lower slopes, and benches in
mesic areas, and (4) Douglas-fir mixed with blue spruce on higher slopes in drainages
and on north-facing slopes. Tree cutting on the study area has not occurred since the
1880s, when a light harvest for railroad ties occurred (Reynolds et al. 1985). Snags
and trees with cavities were relatively abundant throughout the study area (Reynolds
et al. 1985). Elevations ranged from 2,550-2,855 m.
Data Collection and Analysis
47
From 1981-1999, I collected data on the demographic performance of
Flammulated Owls on the study area. Each spring and summer, I searched the entire
study area for territorial males (Marshall 1939). Territories were identified by
marking territorial song-trees of males (Reynolds and Linkhart 1984) and using radio-
telemetry in 1982-1983 (Linkhart et al. 1998). Once territory boundaries were
delineated, I located all suitable nesting cavities (tree cavities with entrance diameters
> 4 cm) within territories and checked each for nesting owls (Reynolds and Linkhart
1984). Unpaired males typically sang throughout a breeding season, whereas singing
in nesting males dramatically declined after egg-hatch (Reynolds and Linkhart 1987).
Because I spent considerable time during each breeding season monitoring singing
males and searching for nests in their territories, I was confident that all nests were
located. Most nests were found during incubation and nests were checked at least
weekly (often two or three times per week) until the young fledged. Breeding adults
were captured at nests (occasionally on perches or day roosts) and banded with U. S.
Fish and Wildlife Service leg bands (Reynolds and Linkhart 1984). I banded owlets
when 2-3 weeks old (fledging occurs at 22-24 days; Reynolds and Linkhart 1987).
Because no nests failed beyond the midpoint of the nestling period (duration of
nestling period was 22-24 d; Reynolds et al. 1987), mean number of fledglings per
brood was identical to mean number of banding-age owlets per brood.
I determined lifetime reproduction for all males and females captured and
recaptured between 1981 and 1999 according to the following criteria. First, I
included only individuals that bred at least once during the study (territorial, non-
breeding males were rarely captured). Second, individuals whose breeding lifespans
48
included 1981 or 1999 (the first and last years of study) were included only if their
total annual breeding attempts were greater than the mean for all individuals whose
known breeding lifespans began after 1981 and terminated before 1998 (henceforth
“inclusive owls”). Mean (+ SE) total breeding attempts for inclusive adults was 2.5 +
0.5 (n = 18) for males and 1.7 + 0.3 (n = 31) for females, so this criterion excluded
seven adults—four males (two males with two breeding attempts and two with one
breeding attempt) and three females (each with one breeding attempt) from LRS
calculations. Third, breeding individuals were excluded from calculations of certain
parameters of lifetime reproduction if their identity was unknown (i.e., they were not
recaptured) in any given year or if their full breeding histories were unknown
(excepting criterion #2 above). Age of first-time breeders could not be determined.
Data on reproductive parameters are therefore conservative if owls nested outside, but
immigrated onto, the study area prior to being banded. This bias may be small given
that I documented just nine cases of breeding dispersal between 1981-1999, with
dispersers usually moving to adjacent territories (Linkhart and Reynolds 1998).
For all individual owls meeting the above criteria, I determined the following
parameters for both sexes: (1) lifetime breeding attempts, defined as the combined
total of successful (i.e., fledged at least one owlet) and failed breeding attempts; (2)
lifetime successful breeding attempts; (3) lifetime production of eggs and fledglings;
(4) relationship between total breeding years and production of fledglings; (5)
relationship between total fledglings and total eggs, (6) contribution to total offspring
by individual adults; and (7) lifetime number of new mates.
Statistical analyses were performed using SAS (SAS Institute 1995). I used
49
Wilcoxon’s test (PROC NPAR1WAY) to evaluate sex differences in reproductive
parameters and effect of breeding experience on brood sizes, and simple linear
regression (PROC GLM) to examine relationships between variables. Means are
presented + standard error (SE), and analyses were considered significant if P < 0.05.
RESULTS
Over the 19 yr study, I recorded 82 breeding attempts (i.e., at least one egg
laid) by 65 adults. I documented the reproductive lives of 59 of these adults: 22
males and 37 females. Except for four males and three females that nested in 1999,
no individuals who nested in prior years on the study area were known to be alive in
1999. Only two individuals (females) returned to breed on the study area (one in
1998 and 1999) after being absent for a breeding season; these females returned to
breed one and two territories distant from original territories (Chapter 4). Unless
otherwise noted, the following are based on these 59 adults.
Lifetime Breeding Attempts
Lifetime reproduction of individuals is the product of mean clutch or brood
size and lifetime breeding attempts. Elsewhere I reported that, over the 19 yr study
for this population, mean clutch size was 2.5 + 0.1 eggs (n = 29, range = 2–4) and
mean brood size for successful nests was 2.4 + 0.1 (n = 67, range = 1-4; Chapter 2).
Total breeding attempts is dependent upon the age of first breeding, number of
breeding attempts per year, and breeding lifespan (i.e., duration of breeding life). I
was unable to determine age of first breeding because adults could not be aged. Most
raptors produce only one brood per year, although several small species of hawk and
falcon and some medium to large owl species renest after initial breeding attempts
50
fail (Newton 1989a, Johnsgard 1988). I never documented any instances of renesting
or multiple broods in Flammulated Owls during the study, even for pairs whose nests
failed early in the breeding cycle. Since this species has a maximum of one breeding
attempt per year, lifetime breeding attempts was equivalent to breeding lifespan.
Most owls in this study bred more than once in their lifetimes, and the mean
total breeding attempts was 2.4 + 0.3 (range = 1 to 10). However, males had
significantly more mean total breeding attempts in their lifetimes (3.0 + 0.5) than
females (2.0 + 0.3; z = 2.38, P = 0.02; Fig. 8). Just 20% of females had three or more
breeding attempts compared to nearly 50% of males (Fig. 8).
Because unsuccessful breeding attempts do not contribute to an individual’s
LRS, I determined total years that owls bred successfully (i.e., produced at least one
owlet). Most owls in the population bred successfully more than once in their
lifetimes, and mean total successful nest attempts for all owls was 2.0 + 0.2 (range =
0 to 9). However, on average males bred successfully more yr (2.5 + 0.4 yr) than
females (1.7 + 0.3 yr; z = 2.46, P = 0.01; Fig. 9). Just 20% of females bred
successfully for > 3 yr compared to nearly 50% of males. These data are similar to
lifetime breeding attempts for both sexes because only 14 nests failed during the
study—82% of all breeding attempts were successful (Chapter 2). The percentage of
individuals that had greater than 80% lifetime nesting success was 73% among males
and 75% among females (Fig. 10).
Lifetime Reproduction
Most owls produced or tended nests that produced > 5 eggs in their lifetimes;
mean total production was 5.7 + 0.7 eggs. However, males tended nests that
51
Figure 8. Lifetime number of nesting attempts by female (light bars) and male (dark
bars) owls.
Per
cent
of i
ndiv
idua
ls
Years
0
10
20
30
40
50
60
70
1 2 3 4 5 6 7 8 9 10
52
Figure 9. Lifetime number of successful nests by female (light bars) and male (dark
bars) owls.
0
10
20
30
40
50
60
70
0 1 2 3 4 5 6 7 8 9
Total successful nests
Per
cent
of i
ndiv
idua
ls
53
Figure 10. Lifetime percentage of successful nests by female (light bars) and male
(dark bars) owls.
Percent of nests successful
Per
cent
of i
ndiv
idua
ls
0
20
40
60
80
100
0-20 21-40 41-60 61-80 81-100
54
produced significantly more mean eggs in their lifetimes (-x = 6.8 + 1.1) than females
produced (-x = 4.9 + 0.8; z = 2.29, P = 0.02; Fig. 11). Less than 30% of females
produced > 6 eggs while nearly 50% of males produced > 6 eggs. Yr of breeding
experience were not associated with larger clutch sizes in either sex; females breeding
for the first time on the study area had mean clutch sizes of 2.5 + 0.1 eggs (n = 29)
and females breeding for > 2 yr had clutches of 2.6 + 0.2 eggs (n = 8; z = 2.16, P =
0.03). Mean clutch size of males breeding for the first time and males breeding for >
2 yr were nearly identical (both produced clutches of 2.6 + 0.2 eggs, n = 14 and 8,
respectively; z = -0.35, P = 0.73).
Owls produced or tended nests that produced a mean 4.8 + 0.6 fledglings over
their lifetimes. As with lifetime production of eggs, however, over their lifetimes
males tended nests that produced more mean fledglings (-x = 6.1 + 1.0) than females
produced (-x = 4.1 + 0.7; z = 2.48, P = 0.01; Fig. 12). Fourteen percent of females
and 5% of males did not produce any fledglings (their nests failed), while 25% of
females and 47% of males produced > 6 fledglings in their lifetimes. Since my data
on lifetime production of eggs and fledglings do not include territorial but non-
nesting males—who comprised approximately half of all territorial males annually
and who may not produce any offspring (Chapter 4)—estimates of LRS parameters
for males may be inflated relative to females.
Previous breeding experience did not affect brood sizes of either sex. Females
breeding for the first time on the study area had mean brood size of 2.1 + 0.2 owlets
(n = 21) while females breeding for > 2 yr had broods of 2.0 + 0.3 owlets (n = 13; z =
0.39, P = 0.69). Males breeding for the first time on the study area had mean brood
55
Figure 11. Lifetime production of eggs by female (light bars) and male (dark bars)
owls.
0
5
10
15
20
25
30
35
40
45
1 3 5 7 9 11 13 15 17 19 21 23 25
Total eggs
Per
cent
of i
ndiv
idua
ls
56
Figure 12. Lifetime production of fledglings by female (light bars) and male (dark
bars) owls.
Total fledglings
Per
cent
of i
ndiv
idua
ls
0
5
10
15
20
25
30
35
1 3 5 7 9 11 13 15 17 19 21 23
57
size of 2.3 + 0.2 owlets (n = 14) while males breeding for > 2 yr years had broods of
2.2 + 0.2 owlets (n = 14; z = 0.73, P = 0.46).
Breeding lifespan (i.e., total years in which owls bred) appeared to be an
important determinant of lifetime reproduction. Breeding lifespan was positively
correlated with total production of fledglings, although the relationship was more
strongly linear for females (r = 0.96, F = 389.4, P < 0.001; Fig. 13A) than for males
(r = 0.87, F = 63.0 P = 0.001; Fig. 13B). Females produced a mean 4.1 fledglings in
2.0 breeding years, and males produced a mean 6.1 fledglings in 3.0 breeding years.
Thus, the more years that Flammulated Owls bred, the more fledglings they
produced. Total owlet production was positively correlated with total egg production
for both sexes (r = 0.97 for females; r = 0.95 for males; Fig. 14A and 14B), and the
strength of the correlation reflected the fact that relatively few nests failed.
The contribution of total offspring by individuals varied greatly for adults of
both sexes. The most productive female produced 18 fledglings over a breeding
lifespan of 8 yr, accounting for 12% of total fledglings produced during the study.
Overall, 17% of females produced 50% of total fledglings (Fig. 15A). The most
productive male tended nests that produced 22 fledglings over a breeding lifespan of
10 yr, accounting for 16% of all fledglings produced during the study. Overall, 27%
of males produced 50% of total fledglings (Fig. 15B).
Lifetime Number of New Mates
Lifetime number of new mates is defined as the total number of unique
partners with which an individual bred over his/her lifetime. Most owls had one or
two new mates in their lifetimes; number of mates for sexes combined was 1.6 + 0.1
58
Fig. 13. Relationship between total breeding years and total fledglings by (A) female
and (B) male owls. Duplicate symbols are omitted from the graph.
B
y = 1.6x + 1.1
R2 = 0.76
0
5
10
15
20
25
0 2 4 6 8 10
y = 2.3x - 0.5
R2 = 0.93
0
5
10
15
20
25
0 2 4 6 8 10
Total breeding years
Total breeding years
Tot
al fl
edgl
ings
T
otal
fled
glin
gs
A
59
Fig. 14. Relationship between total eggs and fledglings for female (A) and male (B)
owls. Duplicate symbols are omitted from the graph.
y = 0.9x + 0.3
R2 = 0.95
0
5
10
15
20
25
0 5 10 15 20 25
B
A
Tot
al fl
edgl
ings
T
otal
fled
glin
gs
Total eggs
Total eggs
y = 0.9x - 0.1
R2 = 0.97
0
5
10
15
20
0 5 10 15 20
A
60
Fig. 15. Percent of total fledglings produced by varying percentages of female (A)
and male (B) owls. Axes showing percent of males and females were based
on ordering individuals from highest to lowest productivity.
0
20
40
60
80
100
0 20 40 60 80 100
0
20
40
60
80
100
0 20 40 60 80 100
Per
cent
of t
otal
fled
glin
gs
(n =
138
) P
erce
nt o
f tot
al fl
edgl
ing
s (n
= 1
34)
Percent of females (n = 35)
Percent of males (n = 22)
B
A
61
(range = 1-5 mates). However, on average males had more mates in their lifetimes
(2.0 + 0.3) than females (1.3 + 0.1; z = -2.28, P = 0.02; Fig. 16). Just 22% of females
had > 2 breeding partners in their lifetimes compared with 50% of males. These
differences are likely associated with greater male longevity on the study area
compared to females (Chapter 2).
DISCUSSION
Reproductive Strategy
Flammulated Owls, which are the second-smallest (mass 50-60 g) North
American strigiform, have a reproductive strategy that resembles larger strigiforms
for which comparable data exist. Male Flammulated Owls have a longer mean
breeding lifespan (3.0 yr) than Tengmalm’s Owls (1.5 yr; Korpimaki 1992) and Barn
Owl (1.2 yr; Marti 1997), despite the fact that Tengmalm’s Owls and Barn Owls have
greater mass by about 200% and 800%, respectively. Mean lifetime production of
fledglings by Flammulated Owls (6.1 fledglings) was somewhat more than
Tengmalm’s Owls (5.4 fledglings; Korpimaki 1992), and Barn Owls (4.7 fledglings;
Marti 1997). For females, Flammulated Owls have a longer mean breeding lifespan
(2.0 yr) than Barn Owls (1.4 yr; Marti 1997), but shorter than the much larger Ural
Owls (4.9 yr; Saurola 1989), whose mass is about 1500% that of Flammulated Owls.
Mean lifetime production of fledglings by females is less for Flammulated Owls (4.1
fledglings) than Barn Owls (6.0 fledglings; Marti 1997) and Ural Owls (8.2
fledglings; Saurola 1989), but more than Eastern Screech Owls (2.9 fledglings;
Gehlbach 1989), whose mass is about 300% of Flammulated Owls.
62
Fig. 16. Lifetime number of new mates for female (light bars) and male (dark bars)
owls.
0
20
40
60
80
100
1 2 3 4 5
Per
cent
age
of in
divi
dual
s
Total new mates
63
Elsewhere (Chapter 2) I reported data that showed, compared to other North
American strigiforms, Flammulated Owls had (1) one of the smallest and least
variable clutch sizes, (2) one of the highest rates of nesting success, and (3)
relatively long lifespan. Coupled with the fact that lifetime production of fledglings
and breeding lifespan is similar to that of several larger strigiforms, and that I never
observed renesting or multiple broods, these data indicate that Flammulated Owls
have a life-history strategy resembling other, larger strigiforms (e.g., Tawny Owls,
Southern 1970; Ural Owls, Saurola 1989; Spotted Owls, S. occidentalis, Forsman et
al. 1984). This strategy contrasts with that of Barn Owls, whose large clutch size,
propensity for multiple clutches annually by some individuals, and short breeding
lifespans resemble many passerine species (Marti 1997).
Individual Variation in LRS
Studies of LRS have revealed three patterns regarding the extent to which
individuals contribute offspring to future generations. First, some individuals that
attempt to breed fail to produce any offspring during their lifetimes (Clutton-Brock
1988, Newton 1989a). Among all birds, the proportion of individuals that attempted
to breed but produced no offspring ranges from 5% (Blue Tits, Parus caeruleus;
Dhondt 1989) to 49% (Barnacle Geese, Owen and Black 1989). In Flammulated
Owls, 14% of females and 5% of males that attempted to breed produced no
fledglings. However, this latter percentage is underestimated, because most unpaired
males, which occupied 30-70% of territories annually (Chapter 4) could not be
captured. The proportion of unpaired males that eventually bred was unknown
(Chapter 4). Second, a small percentage of breeding adults account for the majority
64
of the total fledglings produced in the population (Clutton-Brock 1988, Newton
1989a). Among all birds, the percentage of females that accounted for 50% of total
fledglings ranges from 15% (Red-billed Gulls; Mills 1989) to 31% (Kingfishers;
Bunzel and Druke 1989), and the percentage of males ranges from 14% (Indigo
Buntings; Payne 1989) to 30% (Kingfishers; Bunzel and Druke 1989). In
Flammulated Owls, 17% of females and 27% of males accounted for 50% of total
fledglings. As noted above, the percentage of fledglings accounted for by males is
perhaps inflated. Third, a high proportion of fledglings die before ever attempting to
breed (Clutton-Brock 1988, Newton 1989a). Among all birds, the percentage of
fledglings that die before attempting to breed ranges from 42% (Barnacle Geese;
Owen and Black 1989) to 86% (Blue Tits; Dhondt 1989). I could not determine the
proportion of Flammulated Owls that died before breeding because I could not
distinguish between death and dispersal for fledglings and adults.
Sexual Variation in LRS
In many bird species the extent of sexual differences in variance of LRS is
correlated with the degree of sexual dimorphism (Newton 1989a). Species showing
the least sexual dimorphism exhibit the least sexual variance in LRS, such as in
Florida Scrub Jays (Aphelocoma c. caerulescens; Fitzpatrick and Woolfenden 1989),
while species showing the most sexual dimorphism exhibit greatest differences
between the sexes, with the most dimorphic sex exhibiting the greatest variance in
LRS such as in Red-winged Blackbirds (Agelaius phoeniceus; Orians and Beletsky
1989). Inter-sexual variation in LRS of raptors, which exhibit varying degrees of
reversed sexual size dimorphism (Snyder and Wiley 1976, Mueller 1986), has been
65
studied in only three species: Osprey, Barn Owls, and Flammulated Owls (this
study). Extent of dimorphism is similar among these species (females have 20-30%
more mass than males), but inter-sexual differences in LRS exist only within the two
owl species. Male and female Barn Owls produced a similar mean number of total
eggs and fledglings, but females had longer breeding lifespans than males (Marti
1997). In contrast, mean lifetime production of eggs and fledglings were similar
between male and female Flammulated Owls. However, males had longer breeding
lifespans and more mates over their lifetimes than females. These differences may
result from males having greater longevity than females, which is suggested by an
apparent surplus of males—only about 50% of males breed annually (Chapter 5).
The underlying reasons for differences in longevity are uncertain, but females may
suffer higher mortality than males due to the high cost of egg production immediately
following spring migration (Chapter 2). Alternatively, higher rates of breeding
dispersal by females (Chapter 4) may mean that some females may breed either prior
to, or subsequent to, their arrival on my study area.
Demographic and Ecological Correlates of LRS
My data showed that total breeding years were strongly correlated with
lifetime productivity for female (r = 0.96) and male (r = 0.87) Flammulated Owls,
because clutch sizes varied little and nesting success was high. In fact, breeding
lifespan has emerged as the major demographic determinant of LRS (Newton 1989a).
Among all birds, regression analyses have shown that variance in fledgling
production that was accounted for by breeding lifespan ranged from 29% (Barnacle
Geese; Owen and Black 1989) to 86% (Blue Tits; Dhondt 1989). For some species,
66
other important factors contributing to differences in LRS among individuals were
offspring survival between the egg and fledgling stages and lifetime fecundity
(Newton 1989a).
Few studies have examined the ecological factors associated with individual
variation in LRS. Newton (1989b) reported that LRS of European Sparrowhawks
was greatly influenced by territory quality, where better territories were characterized
as those most frequently occupied annually in a previous study. In my study,
variance in LRS also may have been associated with habitat quality. Males and
females producing the most eggs and fledglings over their lifetimes did so on four
territories (A4, A8, A11, and A29) that were, over the 19 yr study, the most
productive of all territories (Chapter 5). Productivity over all territories was
positively correlated with mature, open stands of ponderosa pine/Douglas-fir and
negatively correlated with Douglas-fir forests, which were denser and consisted of
smaller trees (Chapter 5). Variance in LRS was associated with variable food
abundance in Tengmalm’s Owl (Korpimaki 1988, 1992), nest predation in Merlins
(Falco columbarius; Wiklund 1995), and extreme weather in Great Tits (Parus
major; McCleery and Perrins 1989) and Barnacle Geese (Owen and Black 1989).
Annett and Pierotti (1999) found that long-term reproductive output in Western Gulls
(Larus occidentalis) was strongly influenced by choice of diet, with increasing
amounts of fish in the diet associated with greater survival and reproduction.
67
Literature Cited Annett, C. A. and R. Pierotti. Long-term reproductive output in Western Gulls:
consequences of alternate tactics in diet choice. Ecol. 80:288-297. Bunzel, M. and J. Druke. 1989. Kingfisher. Pages 107-117 in Lifetime reproduction
in birds (I. Newton, Ed.). Academic Press, San Diego, California. Clutton-Brock, T. H. 1988. Reproductive success. University of Chicago Press,
Chicago. Dhondt, A. A. 1989. Blue Tit. Pages 15-33 in Lifetime reproduction in birds (I.
Newton, Ed.). Academic Press, San Diego, California. Fitzpatrick, J. W. and G. E. Woolfenden. 1989. Florida Scrub Jay. Pages 201-218 in
Lifetime reproduction in birds (I. Newton, Ed.). Academic Press, San Diego, California.
Forsman, E. D., E. C. Meslow, and H. M. Wight. 1984. Distribution and biology of
the Spotted Owl in Oregon. Wildl. Monogr. 87:1-64. Gehlbach, F. R. 1989. Screech-owl. Pages 315-326 in Lifetime reproduction in
birds (I. Newton, Ed.). Academic Press, San Diego, California. Johsnsgard, P. A. 1988. North American owls: Biology and natural history.
Smithsonian Institution Press, Washington, D.C. Korpimaki, E. 1988. Costs of reproduction and success of manipulated broods under
varying food conditions in Tengmalm’s Owls. J. Anim. Ecol. 57:97-108. Korpimaki, E. 1992. Fluctuating food abundance determines the lifetime
reproductive success of male Tengmalm’s Owls. J. Anim. Ecol. 61:103-111. Linkhart, B. D. and R. T. Reynolds. 1997. Territories of Flammulated Owls: Is
occupancy a measure of habitat quality? Pp. 250-254 in Biology and conservation of owls of the northern hemisphere (J. R. Duncan, D. H. Johnson, and T. H. Nichols, Eds.). USDA Forest Serv. Gen Tech. Rep. NC-190.
Linkhart, B. D., R. T. Reynolds, and R. A. Ryder. 1998. Home range and habitat of
breeding Flammulated Owls in Colorado. Wilson Bull. 110:342-351. Marshall, J. T., Jr. 1939. Territorial behavior of the Flammulated Screech Owl.
Condor 41:71-78.
68
Marti, C. D. 1997. Lifetime reproductive success in Barn Owls near the limit of the
species range. Auk 114:581-592. McCallum, D. A. 1994. Flammulated Owl (Otus flammeolus). In The birds of North
America, No. 93 (A. Poole and F. Gill, Eds.). Academy of Natural Sciences, Philadelphia; and American Ornithologists’ Union, Washington, D.C.
McCleery, R. H., and C M. Perrins. 1989. Great Tit. Pages 35-53 in Lifetime
reproduction in birds (I. Newton, Ed.). Academic Press, San Diego, California.
Mills, J. A. 1989. Red-billed Gull. Pages 387-404 in Lifetime reproduction in birds
(I. Newton, Ed.). Academic Press, San Diego, California. Mueller, H. C. 1986. The evolution of reversed sexual dimorphism in owls: an
empirical analysis of possible selective factors. Wilson Bull. 98:387-406. Newton, I. 1989a. Lifetime reproduction in birds. Academic Press, San Diego,
California. Newton, I. 1989b. Sparrowhawk. Pages 279-296 in Lifetime reproduction in birds
(I. Newton, Ed.). Academic Press, San Diego, California. Newton, I. 1998. Population limitation in birds. Academic Press, New York. 597
pp. Orians, G. H. and L. D. Beletsky. 1989. Red-winged Blackbird. Pages 183-197 in
Lifetime reproduction in birds (I. Newton, Ed.). Academic Press, San Diego, California.
Owen, M., and J. M. Black. 1989. Barnacle Goose. Pages 349-362 in Lifetime
reproduction in birds (I. Newton, Ed.). Academic Press, San Diego, California.
Payne, R. B. 1989. Indigo Bunting. Pages 153-172 in Lifetime reproduction in birds
(I. Newton, Ed.). Academic Press, San Diego, California. Pianka, E. R. and W. S. Parker. 1975. Age-specific reproductive tactics. Amer.
Natural. 109:453-464. Pietiainen, H. 1970. Breeding season, age, and the effect of experience on the
reproductive success of the Ural Owl (Strix uralensis). Auk 105:316-324. Postupalsky, S. 1989. Osprey. Pages 297-313 in Lifetime reproduction in birds (I.
Newton, Ed.). Academic Press, San Diego, California.
69
Reynolds, R. T. and B. D. Linkhart. 1984. Methods and materials for capturing and
monitoring Flammulated Owls. Great Basin Nat. 44:49-51. Reynolds, R. T., B. D. Linkhart, and J. Jeanson. 1985. Characteristics of snags and
trees containing cavities in a Colorado conifer forest. USDA Forest Serv. Res. Note RM-455. 6 pp.
Reynolds, R. T. and B. D. Linkhart. 1987. The nesting biology of Flammulated
Owls in Colorado. Pp. 239-248 in Biology and conservation of northern forest owls (R. W. Nero, R. J. Clark, R. J. Knapton, and R. H. Hamre, Eds.). USDA Forest Serv. Gen. Tech. Rep. RM-142.
SAS Institute, Inc. 1995. SAS user’s guide, SAS Institute, Inc. Cary, North
Carolina. Saurola, P. 1989. Ural Owl. Pages 327-345 in Lifetime reproduction in birds (I.
Newton, Ed.). Academic Press, San Diego, California. Skutch, A. F. 1949. Do tropical birds rear as many young as they can nourish? Ibis
91:430-455. Snyder, N. F. and J. W. Wiley. 1976. Sexual size dimorphism in hawks and owls of
North America. Ornith. Monogr. 20:1-96. Southwood, T. R. E. 1977. Habitat, the templet for ecological strategies? J. Anim.
Ecol. 46:337-365. Stearns, S. C. 1976. Life history tactics, a review of the ideas. Quart. Rev. of Biol.
51:3-47. Southern, H. N. 1970. The natural control of a population of Tawny Owls (Strix
aluco). J. Zool. (London) 162:197-285. Wiklund, C. G. 1995. Nest predation and life-span: components of variance in LRS
among Merlin females. Ecol. 76:1994-1996. Williams, G. C. 1966. Natural selection, the costs of reproduction, and a refinement
of Lack’s Principle. Amer. Natural. 100:687-690.
70
CHAPTER IV
MATE AND SITE FIDELITY AND BREEDING DISPERSAL IN
FLAMMULATED OWLS
Abstract. - I investigated territory and mate fidelity and dispersal in a migratory
population of Flammulated Owls (Otus flammeolus) on 511 ha in central Colorado. Over
the 19 yr (1981-1999) study, a mean 8.0 + 0.5 (SE) territories were occupied annually by
breeding pairs or unpaired males. Rate of return to the study area was higher for males
(59%) than for females (37%), and mean tenure on territories was nearly twice as long for
males (3.0 + 0.5 yr) as for females (1.6 + 0.2 yr). Males also showed greater annual
territory fidelity than females; 98% of returning males stayed on original territories
compared to 78% of females. Failure of previous year’s nest and breeding status (paired
vs unpaired) were associated with reduced territory fidelity in females but not males,
while return of previous mate did not affect territory fidelity for either sex. Mean pair
duration was 1.4 + 0.1 yr and mate fidelity was high; 96% of pairs retained the same mate
when both mates returned from migration. Reproductive success was not correlated with
length of pair bond; annual initiation of egg-laying and productivity (mean
fledglings/brood) were similar for pairs that bred two or more years and pairs that bred for
the first time. I documented one instance of natal dispersal. Breeding dispersal was
female-biased; 8 of 9 dispersals were by females that moved one or two territories away
from their original territories. Females whose nests failed the previous year had lower
return rates to the study area than females whose previous nests were successful.
71
Consequently, dispersal distance may be bimodal with females dispersing longer distances
after nesting failure and shorter distances after successful nests. Dispersal was not
correlated with previous breeding experience, previous nest failure, mate quality, and
productivity, but females often dispersed when mates did not return and they dispersed to
territories on which total productivity over the study was higher than on original
territories.
72
INTRODUCTION
In stable environments many birds annually return to the same breeding sites
and retain the same breeding partners (Greenwood 1980, Black 1996). Particularly
for migratory species, site fidelity may confer benefits that include better knowledge
of locations for foraging, nesting, and escaping predators, and improved chance of
maintaining a breeding territory and mating (Hinde 1956, Greenwood 1980, Shields
1984, Part 1994, 1995). If neighbors return, site-faithful individuals also benefit by
avoiding initial cost of contesting territory boundaries with unfamiliar individuals
(Krebs 1982, Maynard Smith 1982). Mate fidelity is thought to improve coordination
and cooperation between mates, prolong biparental investment, and reduce costs
associated with mate sampling such as risk of predation or failing to find a suitable
mate in time to breed (Black 1996). Studies have shown that mates who bred
together previously had larger clutches and higher nest success than newly formed
pairs (Mills 1973, Coulson and Thomas 1983, Newton and Marquiss 1982,
Korpimaki 1988, Bradley et al. 1990, Orell et al. 1994, Murphy 1996).
In spite of benefits, mate and site fidelity involve some costs. Site faithfulness
increases the inbreeding probability, and may lower chances for reproductive success
due to decrease in habitat quality resulting from habitat changes (i.e., succession or
disturbance), predation, or competition (Greenwood et al. 1978, Oring and Lank
1982). For migratory species, costs of mate fidelity may include waiting for mates to
return or searching for new mates (Black 1996). When costs associated with mate or
site fidelity exceed benefits, individuals should disperse. In fact, several studies have
shown that fitness of individuals increased following breeding dispersal, when adults
73
moved to new breeding sites (Shields 1984, Payne and Payne 1993, Part 1995, Forero
et al. 1999).
Despite much study the past two decades, avian fidelity and dispersal are still
poorly understood primarily because investigations require gathering longitudinal
data on marked individuals and studying sufficiently large areas to document longer-
distance dispersal (Paradis et al. 1998, Forero et al. 1999). These problems are the
primary reasons that raptors, which are relatively long-lived and disperse over large
areas, have been little-studied. Diurnal species studied five or more years include
Merlins (Falco columbarius; Warkentin et al. 1991), European Sparrowhawks
(Accipiter nisus; Newton and Marquiss 1982, Newton 1993), and Black Kites (Milvus
migrans; Forero et al. 1999), and nocturnal species include Tengmalm’s Owl
(Aegolius funereus; Korpimaki 1988, 1993), Flammulated Owls (O. flammeolus;
Reynolds and Linkhart 1987a), Eastern Screech Owls (Otus asio; Gelhbach 1994),
and Barn Owls (Tyto alba; Marti 1999). Of these species, only Black Kites and
Flammulated Owls are migratory. Because dispersal influences population structure
and dynamics (e.g., Greenwood and Harvey 1982, Johnson and Gaines 1990),
understanding the ecological correlates of dispersal is important for avian
conservation.
Here I describe fidelity and dispersal for a Colorado population of
Flammulated Owls, which are among the most migratory of North American
strigiforms (Johnsgard 1988). I extend the study by Reynolds and Linkhart (1987a)
to include 19 yrs of data (1981-1999) on the same owl population. Specifically, I
describe: (1) sex differences in return rates to the study area; (2) sex differences in
74
territory fidelity, and I evaluate effects of previous nest failure, breeding status
(paired vs unpaired), and return of previous mate on fidelity; (3) patterns in mate
fidelity and I assess benefits of maintaining pair bonds; and (4) sex differences in
breeding dispersal. I also test possible correlates of breeding dispersal: (a) owls
dispersed due to failure of previous nest attempt (nest-failure hypothesis); (b) owls
dispersed because original mates did not return (non-return of mate hypothesis); (c)
owls that dispersed were inexperienced breeders (inexperienced-breeder hypothesis);
(d) owls were able to acquire higher quality mates (mate-improvement hypothesis);
(e) owls were able to acquire higher quality territories (territory-improvement
hypothesis); and (f) owls were able to increase their productivity (disperser-
enhancement hypothesis).
METHODS
Species
Flammulated Owls are insectivorous, feeding mostly on small lepidopterans
(Reynolds and Linkhart 1987b). The owls are obligate cavity-nesters that breed in
montane forests of western North America as far north as southern British Columbia
and winter as far south as El Salvador (McCallum 1994, Linkhart et al. 1998). They
are “sensitive species” in four of six regions of the USDA Forest Service (Verner
1994). Flammulated Owls are monogamous although extra-pair copulations are
known (Reynolds and Linkhart 1990); females are sole incubators of eggs while
males defend territories and provide food for their mates and fledglings (Reynolds
and Linkhart 1987b, Linkhart et al. 1998). Nest success by Flammulated Owls is
75
among the highest (82%) and clutches are among the smallest (2.5 + 0.5 eggs) of
North American strigiforms (Chapter 2; Johnsgard 1988).
Study Area
The study was conducted on the Manitou Experimental Forest in Teller Co.,
Colorado. I established boundaries of the 511 ha study area after territorial owls were
found to be present in initial surveys (1980). Boundaries were drawn around an area
large enough to contain approximately 20 territorial males, based on an estimate of
territory size (274 m in diameter; Marshall 1939). Forests within the study area
consist of (1) ponderosa pine (Pinus ponderosa) mixed with Douglas-fir
(Pseudotsuga menziesii), generally on ridgetops and south- and west-facing slopes,
(2) quaking aspen (Populus tremuloides) stands on lower slopes and bottoms of moist
drainages, (3) quaking aspen stands mixed with blue spruce (Picea pungens) in
bottoms, lower slopes, and benches in mesic areas, and (4) Douglas-fir mixed with
blue spruce on higher slopes in drainages and on north-facing slopes. Tree cutting on
the study area has not occurred since the 1880s, when a light single-tree selection cut
for railroad ties occurred (Reynolds et al. 1985). Snags and trees with cavities are
relatively abundant throughout the study area (Reynolds et al. 1985). The forest
understory, consisting of over 100 species of grasses, forbs, and shrubs, is poorly
developed in all but the moist creek bottoms (Reynolds et al. 1985). Terrain is
moderately steep (20-80% slope) and elevations ranged from 2,550-2,855 m. The
study area is surrounded by forests composed of a similar mix of forest types and
ages.
Locating and Capturing Owls
76
Each spring and summer from 1981-1999, I searched the entire study area for
territorial males (Reynolds and Linkhart 1984). I identified territory boundaries by
marking territorial song-trees of males (Reynolds and Linkhart 1984) and using radio-
telemetry in 1982-1983 (Linkhart et al. 1998). I located all suitable nesting cavities
(tree cavities with entrance diameters > 4 cm) within territories and checked each for
nesting owls (Reynolds and Linkhart 1984). Unpaired males typically sang
throughout a breeding season, whereas singing by paired (nesting) males dramatically
declined after egg-hatch (Reynolds and Linkhart 1987b). Due to extensive nest
searching throughout each summer I was confident that all nests were located. Most
nests were found during incubation (late May and early June) and all nests were
checked weekly (most often two or three times per week) until the young fledged
(mid July). Breeding adults were captured at nests (occasionally on perches or day
roosts) and banded with U. S. Fish and Wildlife Service leg bands (Reynolds and
Linkhart 1984). I determined sex of adults by behavioral and morphological
characteristics (see Reynolds and Linkhart 1984). I banded owlets when they were 2-
3 weeks old (fledging occurs at 22-24 days; Reynolds and Linkhart 1987b). A total
215 Flammulated Owls were banded on the study area from 1981-1999, including
146 owlets (sex unknown) and 69 adults (29 males and 40 females). Because
unpaired males were difficult to capture, all banded males but one bred at least once.
Unless otherwise noted, data on territory and mate fidelity and natal and breeding
dispersal were determined from annual capture-recapture data on all adults banded
from 1981 to 1999.
Return Rate, Turnover, and Fidelity to Territory and Mate
77
I determined sex-specific return rates to the study area by dividing the number
of banded males and females that were recaptured for two or more consecutive years
by total number of banded owls (excluding one male and two females banded in
1999). I defined territory tenure as total years an owl occupied the same territory. I
defined turnover as the replacement of a marked individual on a territory by another.
Turnover was calculated for each sex by dividing the number of occasions in which a
marked individual was known to have been replaced on a territory by the total
number of occasions (opportunities) in which identities of individuals on a territory
were known in consecutive yr. Territory fidelity, reoccupancy of the same territory
by the same owl for consecutive years, was calculated by dividing total years in
which banded owls returned to original territories by total owl-years (count of years
in which I could ascertain that males or females returned to original territories or
dispersed to new territories). I presumed a banded male on a territory in year X was
present on the same territory in year X + 1, despite not recapturing him in year X + 1,
if: (1) a male was heard singing on the territory in year X + 1; and (2) the original
male was recaptured in a subsequent year on the same territory. This assumption,
which accounted for 7 different males, was based on the fact that there were no
instances of a male leaving a territory and returning in a subsequent year to the same
territory. I presumed a banded female that nested in year X on a territory did not
return to the same territory in year X + 1 if she was not seen or if no nest was found
that year, since there were no instances of a female failing to nest in one year but
nesting on the same territory in the previous year and subsequent year.
To determine if previous nesting failure affected territory fidelity, I compared
78
return of owls to original territories in year X + 1 following nest success in year X to
return of owls to original territories following nesting failure in year X. To determine
if breeding status affected territory fidelity, I compared return of owls that were
paired (new or original mate) in year X to return of owls that were unpaired in year X.
Finally, to determine if return of previous mate affected territory fidelity, I compared
return of owls to original territories when original mates also returned to original
territories to return of owls to original territories when original mates did not return to
original territories. I omitted cases of known breeding dispersal when evaluating
effects of the above factors on territory fidelity. I could not distinguish between
mortality and dispersal beyond the study area for owls that did not return to the study
area.
I defined pair duration as total years in which the same male and female
remained paired, and mate fidelity as the same male and female remaining paired for
multiple years. Mate fidelity was calculated by dividing total years in which the same
male and female remained paired for subsequent years by total pair-years (total years
in which both pair members were known to be alive after migration). Divorce
occurred when both pair members from a particular year were known to be alive in a
subsequent year but one or both had different mates (sensu Rowley 1983). I
calculated divorce rate by dividing total divorces by total pair-years. I assessed affect
of pair duration on reproductive success by comparing initiation of incubation and
productivity (i.e., mean owlets per brood) for pairs breeding for the first time and
pairs that bred two or more years. Initiation of incubation was determined either by
observing nest contents and female behavior during egg-laying and incubation, or by
79
back-dating from hatching (mean duration of incubation was 22 days; Reynolds and
Linkhart 1987) if nests were discovered after females began incubating.
My estimates of territory tenure, territory fidelity, pair duration, and mate
fidelity included adults banded in 1981 (n = 9 unknown histories) and adults banded
and/or recaptured in 1999 (n = 10 unknown futures). In 1981 I banded 4 males (one
returned for 3 yr, one for 2 yr, and two for 1 yr) and 5 females (one returned for 7 yr,
one for 4 yr, one for 3 yr, and two for 1 yr), and in 1999 I banded or recaptured 5
males (one had returned for 12 yr, one for 8 yr, one for 5 yr, one for 2 yr, and one for
1 yr) and 5 females (three had returned for 2 yr and two for 1 yr). Mean total years of
return for “fringe” males whose histories included 1981 or 1999 (3.9 + 1.3 yr) was
actually greater than “inclusive” males whose known breeding lifespans began after
1981 and terminated before 1998 (2.6 + 0.5 yr, n = 22), although the difference was
not significant (t = -0.94, df = 11, P = 0.37, two-tailed). Likewise, mean total years
of return for fringe females (2.4 + 0.6 yr) was greater than those of inclusive females
(1.7 + 0.3 yr, n = 31), but the difference also was not significant (t = -1.03, df = 14, P
= 0.32, two-tailed). In 1981 I identified 5 pair bonds (pair duration for one was 3 yr,
one was 2 yr, and for three was 1 yr), and in 1999 I identified 5 pair bonds (pair
duration for two was 2 yr and for three was 1 yr). Mean duration of fringe pair bonds
(1.6 + 0.2 yr) was greater than inclusive pair bonds (1.4 + 0.1 yr, n = 44), but not
significantly (t = -0.72, df = 12, P = 0.49, two-tailed).
Natal and Breeding Dispersal
I defined natal dispersal as movement from birth territory to breeding
territory, and breeding dispersal as movement between territories by breeding owls
80
(sensu Greenwood and Harvey 1982). Because Flammulated Owls nested a
maximum of once per breeding season (Linkhart et al. 1998, Chapter 2), breeding
dispersal refers to owls changing territories between years. Breeding dispersers may
or may not have retained the same mate. Breeding dispersal distances were straight-
line distances between nest trees in successive territories, and were measured in
Arcview (ESRI 1995).
I evaluated six correlates of breeding dispersal, two of which were possible
causes of dispersal. To test a nest-failure hypothesis, I determined if breeding
dispersal in yr X + 1 was preceded by nest failure in yr X. To test a non-return of
mate hypothesis, I determined if dispersal occurred when original mates did not return
to original territories. To test an inexperienced-breeder hypothesis, I compared the
proportion of dispersers that had one yr of breeding experience to the proportion with
> 2 yr of breeding experience. I could not determine whether owls bred elsewhere
before breeding on the study area. To test a mate-improvement hypothesis, I defined
quality of mates by their lifetime reproductive success (LRS), a good estimate of
individual fitness (Williams 1966, Newton 1989), and compared LRS of new mates
of dispersers with LRS of original mates. To test a territory-improvement hypothesis,
I defined territory quality by total owlets produced on a territory over the 19 yr study.
Thus, I compared total owlets produced on original territories of dispersers to total
owlets produced on new territories. Finally, to test a disperser-enhancement
hypothesis, I compared mean brood size of dispersers on original territories to mean
brood size on new territories.
Statistical analyses were performed using Statistical Analysis System (SAS
81
Institute 1995). I used Wilcoxon’s test (PROC NPAR1WAY) to evaluate sex
differences in total yr of return, and Fisher’s exact test (PROC FREQ) and Chi-square
contingency table analyses to evaluate sex differences in return rates and territory
fidelity, and effect of factors on territory fidelity. I used Wilcoxon and unpaired t-
tests assuming unequal variances (PROC TTEST) to evaluate differences between
sexes for fidelity and dispersal parameters, and to evaluate dispersal hypotheses; all
tests were two-tailed. Analyses were considered significant if P < 0.05. Throughout
the paper I report means + SE.
RESULTS
Territory Occupancy and Return Rate
Over the 19 yrs I determined fidelity and dispersal parameters of owls on 14
territories (Fig. 17). Each year a mean 8.1 + 0.5 territories were occupied by owls;
breeding pairs occupied a mean 4.5 + 0.2 territories annually while non-breeding
males occupied 3.6 + 0.5 territories (Chapter 2). Most territories remained fixed over
years; boundaries of territories shifted little despite turnover of owls on territories
(Chapter 5).
Annual return of adult Flammulated Owls from spring migration appeared to
be sex-biased, as in many migratory birds (Greenwood 1980, Ens et al. 1996). Males
settled into territories between 1-15 May. Females settled in territories over a longer
time period; in some years females appeared on territories as early as the first week of
May or as late as mid-June, but most were detected on territories between 10-20 May.
For sexes combined, rate of return to the study area for two or more consecutive years
82
Figure 17. Location of owl territories (black polygons) on the Manitou Experimental
Forest study area (white boundary), 1981-1999.
83
was 0.46 (31 of 67); return rate for males was 0.59 (17 of 29) and return rate for
females was 0.37 (14 of 38; Fisher’s exact test, P = 0.09). Only two owls (both
females) returned to breed on the study area after each was undetected for one
breeding season (one in 1998 and 1999).
Territory Fidelity
Mean tenure on territories for all adults was 2.1 + 0.2 yr (n = 78) but varied by
sex. Mean tenure was greater for males (3.0 + 0.5 yr, n = 31) than for females (1.6 +
0.2 yr, n = 47; z = 2.47, P = 0.02). Thus, mean tenure of males on territories was
nearly twice mean tenure for females. Thirty-nine percent (12 of 31) of males
occupied the same territory for > 3 yr, compared to just 13% (6 of 47) of females.
As data on tenure indicate, turnover on territories was female biased. Annual
turnover rate for females was 37% (16 of 43 opportunities) while turnover rate for
males was just 10% (7 of 69 opportunities; Fisher’s exact test, P = 0.001). Each year
there were 1-6 opportunities to assess turnover of each sex on breeding territories; 1
or 2 females were replaced on breeding territories in 15 of 19 yr while < 1 male was
replaced in any year and no males were replaced during the last 9 yr of the study
(1991-1999).
Calculations of territory fidelity were based on 62 male-years and 36 female-
years. Territory fidelity by both sexes was 91%. However, males were significantly
more faithful to territories than females; fidelity by males was 98% (61 of 62) while
fidelity by females was 78% (28 of 36; χ2 = 11.6, df = 1, P = 0.001). Estimates of
territory fidelity for both sexes may be overestimated given the high proportion of
territories that occurred near study boundaries. Fifty percent (7 of 14) of territory
84
centers were within one mean territory-diameter (439 + 77 m; Chapter 5) of a study
boundary and 100% were within two mean territory-diameters of a study boundary
(Fig. 17), suggesting that some individuals may have dispersed undetected.
Failure of previous year’s nest did not affect territory fidelity in males; return
to original territories following success of previous year’s nest was 76% (38 of 50)
while return following failure of the previous year’s nest was 78% (7 of 9). However,
54% (26 of 48) of females returned to original territories following a successful nest
while only 9% (1 of 11) returned following failure of the previous year’s nest
(Fisher’s exact test, P = 0.008). Thus, while the latter sample size was small these
data suggest that effect of nest failure on territory fidelity was female-biased.
Breeding status (paired or unpaired) appeared to affect territory fidelity of
females but not males. Seven males continued to occupy their original territories
despite being unpaired for 4 consecutive yr (1 male), 3 consecutive yr (1 male), 2
consecutive yr (1 male), and 1 yr (4 males). In contrast, while detecting females was
more difficult than males because females did not defend territories, I documented no
instances in which females remained unpaired during a breeding season.
Return of previous mate did not affect territory fidelity in either sex. For
males, return to original territories when previous mates returned was 70% (23 of 33)
and return to original territories when previous mates did not return was 83% (19 of
23; χ2 = 1.21, df = 1, P = 0.27). For females, return to original territories when
previous mates also returned was 51% (23 of 45) and return to original territories
when previous mates did not return was 40% (4 of 10; χ2 = 0.4, df = 1, P = 0.53).
Mate Fidelity
85
I recorded 82 nesting attempts (i.e., at least one egg laid) by 53 unique pairs
involving 47 females and 31 males. Mean pair duration was 1.4 + 0.1 yr; 74% (39 of
53) of pair bonds endured one year, 15% (8 of 53) endured two years, 9% (5 of 53)
endured three years, and 2% (1 of 53) endured four years. However, faithfulness to
mates was high; 96% of pairs (22 of 23 pair-years) retained the same mate from one
nesting attempt to the next. Only one divorce occurred in 19 yr. The divorced female
nested successfully with her original mate in 1997 and was found nesting with a new
male two territories distant in 1999. Although I did not locate her breeding in 1998,
her original male nested in 1998 and 1999 with a new female. I was unable to
determine cause of the divorce.
I assessed effect of pair duration on two measures of reproductive success,
initiation of incubation and productivity (i.e., mean owlets per brood), which were
associated with pair duration in other birds (e.g., Newton and Marquiss 1982,
Korpimaki 1988). Pairs that bred for the first time initiated incubation only 3 days
later (5 June + 1 day, n = 26, range = 27 May-20 June) than pairs that bred two or
more years (2 June + 2 days, n = 16, range 20 May-13 June), and this difference was
not significant (t = 1.31, df = 34, P = 0.20). First-year pairs had only slightly fewer
mean owlets per brood (2.2 + 0.1 owlets, n = 29, range 0-3) than pairs that bred two
or more years (2.4 + 0.2 owlets, n = 20, range 0-4), and again this difference was not
significant (t = -0.40, df = 33, P = 0.69). Thus, mates that bred together two or more
years did not appear to have significant reproductive advantages over pairs breeding
for the first time.
86
Natal Dispersal
I documented one instance of natal dispersal; a male owlet banded on the
study area in 1981 was found breeding on another territory 1.4 km distant in 1987.
This male also bred in 1988 and 1989 with a different female in each year, and
produced five total owlets in three successful nest attempts (Chapter 3). Whether this
male bred in years prior to 1987 outside of the study area was unknown.
Breeding Dispersal
I documented 9 instances of breeding dispersal, including one pair, in 98 bird-
years for a dispersal rate of 0.092. Females dispersed more frequently than males;
females moved to new territories on 8 occasions (in 36 female-years) for a dispersal
rate of 0.22, while just 1 male (in 62 male-years) moved to a new territory for a
dispersal rate of 0.02. Mean dispersal distance for all owls (570 + 52 m, n = 9) was
approximately 1.3 times mean diameter of territories (439 + 77 m; Chapter 5); 78% (7
of 9 owls, including the pair) dispersed to adjacent territories while 22% (2 of 9)
dispersed two territories away from original territories. Mean dispersal distance for
females was 580 + 58 m; one female dispersed 375 m, four dispersed 400-599 m,
three dispersed 600-799 m, and one dispersed 845 m. Dispersal distance for the one
male was 480 m. Given the high proportion of territories that occurred near study
boundaries (see Territory Fidelity), estimates of breeding dispersal rate and distance
may be underestimated.
87
Correlates of Breeding Dispersal
Nest-failure Hypothesis.—Eighty-eight percent of females (7 of 8) and the one male
dispersed after nesting successfully the previous year. Thus, failure of previous nest
was not associated with breeding dispersal.
Non-return of Mate Hypothesis.—In all cases where identity of mates was known,
female dispersal occurred when original males did not return. Sixty-seven percent of
females (4 of 6, excluding the divorced female and the female that dispersed with her
original male) dispersed when their original males did not return to original
territories. In the two remaining instances of female dispersal, unidentified males that
were unpaired occupied original territories. However, females did not always
disperse when original males did not return, based on the fact that 4 of 6 (67%) other
females did not disperse when original males failed to return. Thus, these data
suggest that factor(s) other than non-return of mate were involved in dispersal.
Breeding-experience Hypothesis.—Dispersers generally had > 1 yr of breeding
experience; 63% (5 of 8) of females had > 2 yr of breeding experience on original
territories and the one male had 3 yr of breeding experience. Thus, most owls that
moved to new territories were not inexperienced breeders.
Mate Improvement Hypothesis.—LRS of new mates on new territories was higher
than original mates on original territories, although not significantly. LRS of original
mates was 5.0 + 1.0 owlets (range 2 to 8) while LRS of new mates after dispersal was
7.1 + 0.9 owlets (range 2 to 9; t = -1.58, df = 12, P = 0.14; Table 1). The pair that
dispersed together was excluded from this analysis.
Territory Improvement Hypothesis.—Females dispersed to territories of higher long-
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Table 1. Mate improvement hypothesis; comparison of lifetime production of owlets
by original males on territories from which females dispersed to lifetime production
of owlets by new males on territories where females dispersed.
Original Male New Male
Territory
(yr)
Lifetime
Owlets
Territory
(yr)
Lifetime Owlets
A15 (1982) 3 A4 (1983) 2
A15 (1984) 2 A12 (1985) 7
A12 (1986) --1 A12 (1986) --1
A29 (1983) 8 A12 (1984) 9
A24 (1983) 4 A29 (1984) 9
A7 (1990) 3 A29 (1991) 9
A13 (1996) 7 A8 (1998) 7
A10 (1997) 8 A8 (1999) 7
MEAN 5.0 MEAN 7.1
SE 1.0 SE 0.9
1 Female was excluded because she had the same mate before and after dispersal.
89
term productivity; territories where females originally nested produced a mean of 9.0
+ 1.8 owlets over the study while territories to which females dispersed produced a
mean of 20.5 + 3.8 owlets (t = -2.53, df = 7, P = 0.04; Table 2). In the one case of
male dispersal, movement was to a territory whose long-term productivity (n = 35
owlets) over the study was greater than on the territory where he originally bred (n =
7 owlets).
Disperser-enhancement Hypothesis.—Overall, dispersers did not attain higher
productivity on territories following dispersal. For females, mean brood size on
territories before they dispersed was 2.3 + 0.2 owlets, and mean brood size on
territories after they dispersed was 2.3 + 0.2 owlets (t = 0.25, df = 10, P = 0.81; Table
3). Mean brood size for the one dispersing male was 2.3 + 0.4 owlets (n = 3 nests)
before dispersal and 2 owlets (n = 1 nest) after dispersal.
DISCUSSION
Territory Fidelity
Male Flammulated Owls had significantly longer tenure on territories than
females, which probably resulted from males having greater territory fidelity than
females (98% vs 78%), and an apparently longer male lifespan. Male-biased site
fidelity is widespread among birds (e.g., passerines: Shields 1984, Murphy 1996;
shorebirds: Mills et al. 1996, Sydeman et al. 1996; raptors: Newton and Marquiss
1982, Korpimaki 1988, Forero et al. 1999), perhaps because in resource-defense
mating systems (Emlen and Oring 1977) males have more to gain by being faithful to
breeding territories than females (Greenwood 1980). This may be particularly true in
90
Table 2. Territory improvement hypothesis; comparison of total owlets produced
over 19 yr on territories where females nested before dispersal (“original territory”) to
total owlets produced on territories where females dispersed (“new territory”).
Original Territory New Territory Net Change
in
Territory
(yr)
Total Owlets Territory (yr) Total Owlets Owlets (%)
A15 (1982) 7 A4 (1983) 35 400
A29 (1983) 17 A12 (1984) 7 -59
A24 (1983) 4 A29 (1984) 17 325
A15 (1984) 7 A12 (1985) 7 0
A12 (1986) 7 A4 (1987) 35 400
A7 (1990) 3 A29 (1991) 17 467
A13 (1996) 14 A8 (1998) 23 64
A10 (1997) 13 A8 (1999) 23 77
MEAN 9.0 MEAN 20.5 MEAN 209
SE 1.8 SE 3.8
91
Table 3. Disperser-enhancement hypothesis; comparison of mean brood size for
females on territories before they dispersed (“original territory”) to mean brood size
on territories after they dispersed (“new territory”).
Original Territory New Territory
Territory (yr) Mean Brood
Size (N)
Territory (yr) Mean Brood
Size (N)
A15 (1981-2) 2.5 + 0.5 (2) A4 (1983) 2 (1)
A15 (1984) 2 (1) A12 (1985-6) 2.5 + 0.5 (2)
A12 (1985-6) 2.5 + 0.5 (2) A4 (1987-90) 2.5 + 0.3 (4)
A29 (1981-3) 2.7 + 0.4 (3) A12 (1984) 2 (1)
A24 (1981-3) 1.3 + 0.7 (3) A29 (1984) 3.0 + 0 (3)
A7 (1990) 3 (1) A29 (1991) --1
A13 (1995-6) 2.0 + 0 (2) A8 (1998) 3 (1)
A10 (1997) 3 (1) A8 (1999) 0 (1)
MEAN 2.3 MEAN 2.3
SE 0.2 SE 0.2
N 15 N 13
1 Brood size was unknown.
92
raptors such as Flammulated Owls where males are the primary foragers. While
longer lifespan in males has not been substantiated, it is plausible for two reasons.
First, mean longevity on the study area from 1981-1999 was significantly greater for
males than females (3.2 + 0.6 yr vs 2.0 + 0.3 yr; Chapter 2), although this disparity
may in part result from female-biased dispersal (see below). Second, unpaired males
annually occupied 10-70% of territories, which suggests a shortage of females in the
breeding population and may reflect a longer mean lifespan by males (Chapter 2).
Other studies of monogamous species have reported or suggested male-biased sex
ratios in adults (Shields 1984, Breitwisch 1989 and sources therein, Burke and Nol
1998, Gibbs and Faaborg 1990, Payne and Payne 1990, Murphy 1996, but see
Kenward 1999).
Breeding Dispersal
Female Flammulated Owls had a higher breeding dispersal rate than males
(0.22 vs 0.02), a pattern also found in many other birds including raptors (e.g.,
Greenwood 1980, Marquiss and Newton 1982, Forero et al. 1999). In all cases of
female dispersal except two (the one divorce excluded) I was able to determine that
the original male did not return. However, 4 of 6 (67%) females whose mates were
known not to return remained on original territories, suggesting that other factors
were involved. Dispersal by females when mates do not return is likely adaptive
because opportunity to breed may be lost while waiting for a new male, especially
since most females apparently return from spring migration after males. Individuals
of other species, including raptors, also dispersed when mates did not return
93
(Greenwood and Harvey 1976, Newton and Marquiss 1982, Warkentin et al. 1991,
Montalvo and Potti 1992, Forero et al. 1999).
Frequency and distance of breeding dispersal can be underestimated in small
study areas (Barrowclough 1978, van Noordwijk 1984, Koenig et al. 1996), as they
may have been in my study. Owls dispersed one or two territories away from original
territories, but half (7 of 14) of all territories were within one mean territory-diameter
(439 + 77 m), and all territories were within two mean territory-diameters, of a study
boundary (Fig. 17). Thus, owls that may have dispersed to territories farther than two
territories were less likely to be detected. Moreover, females (but not males) whose
nests failed the previous year had lower return rates (9%) to the study area than
females whose previous nests were successful (54%), suggesting either higher
mortality or higher dispersal (beyond the study area) among females whose nests
failed. Females whose prior nests failed did not appear to be in poorer physical
condition than females whose nests were successful. That females dispersed rather
than died following failed nests is supported by Haas (1998), who found that
American Robins (Turdus migratorius) and Brown Thrashers (Toxostoma rufum)
subjected to experimental nesting failure returned at significantly lower rates than
birds that had nested successfully. Indeed, other studies have found that following
nesting failure females returned less frequently to territories or they dispersed farther
than males (e.g., Shields 1984, Gavin and Bollinger 1988, Payne and Payne 1993,
Beletsky and Orians 1991, Murphy 1996).
Consequently, dispersal distance by female Flammulated Owls may be
bimodal with females dispersing longer distances following unsuccessful nests and
94
shorter distances following successful nests. Dispersal to nearby territories, as found
in many other studies (e.g., Payne and Payne 1996, Hannon and Martin 1996,
Williams 1996, Forero et al. 1999), may be beneficial because dispersers can best
judge quality of resources and owls in adjacent territories (Hinde 1956, Greenwood
1980, Ens et al. 1996). Means by which owls assess territories or mates for potential
future occupancy or pairing is uncertain but may be accomplished by extra-territory
movements by males and females (Reynolds and Linkhart 1990, BDL, unpubl. data).
Although dispersal following nesting failure is beneficial if chances of future nesting
success are improved (Murphy 1996), benefits of dispersing to more distant
territories, where owls are unlikely to have knowledge of resources or potential
mates, are not clear.
Dispersing owls moved to territories where productivity over the 19 yr study
was significantly greater than on territories from which they dispersed. Since mean
brood size of owls after they dispersed did not increase on new territories, and
because mean total owlets produced by new mates was not significantly greater than
total owlets produced by original mates, my ability to detect consequences of
breeding dispersal may require reproductive data collected over lifetimes for
particular individuals. I reported previously that long-term productivity and
occupancy of territories by breeding pairs was positively correlated with percentage
of old ponderosa pine/Douglas-fir forests and negatively correlated with percentage
of young Douglas-fir/blue spruce forests, and that old ponderosa pine/Douglas-fir
forests were used significantly more for foraging by radio-tagged males (Linkhart and
Reynolds 1997, Linkhart et al. 1998, Chapter 5). Thus, owls dispersed to territories
95
containing more old ponderosa pine/Douglas-fir and less young Douglas-fir/blue
spruce. Other raptors and passerines also dispersed to higher-quality territories, as
inferred by movements to territories having higher historical nesting success, better
prey resources, or lower risk of predation (Baeyens 1981, Marquiss and Newton
1982, Weatherhood and Boak 1986, Beletsky and Orians 1987, Matthysen 1990,
Forero et al. 1999).
Several avian studies have found that younger individuals were more likely to
disperse than older individuals (e.g., Beletsky and Orians 1987, Payne and Payne
1993, Badyaev and Faust 1996). However, breeding dispersal was not associated
with younger owls in this study. Most (5 of 8 females and 1 male) dispersing
Flammulated Owls had at > 2 yr of breeding experience.
Mate Fidelity
The high mate fidelity in Flammulated Owls contrasts with the pattern of
lower mate fidelity in most other migratory birds, which presumably occurs because
of difficulty maintaining pair bonds during migration and asynchronous arrival times
on breeding grounds compared to residents (Wickler and Seibt 1983, Murphy 1996).
That I documented only one divorce in this study suggests that mate (and possibly
territory) familiarity conferred advantages to breeding Flammulated Owls. Studies of
raptors and other birds reported that mate fidelity was associated with higher
productivity, nest success, or earlier nesting (e.g., Newton 1982, Korpimaki 1988,
Orell et al. 1994, Murphy 1996), although other species showed no apparent benefit
by being faithful to mates (e.g., Freed 1987, Warkentin et al. 1991). Flammulated
Owls paired together for > 2 yr did not initiate incubation significantly earlier nor
96
were broods significantly larger than pairs breeding for the first time, although I could
not determine if owls gained breeding experience before their tenure on the study
area. Alternatively, owls may have long-term pair bonds simply because they are
constrained from other alternatives (Freed 1987); male choice appears to be restricted
by female availability while female choice may be limited to males available in
neighboring territories with previous breeding experience. High mate fidelity also
may be facilitated by high territory fidelity by males (Murphy 1996). Indeed, mean
tenure of males on territories (3.0 + 0.5 yr) was nearly twice mean tenure of females
on territories (1.6 + 0.2 yr), suggesting that returning females were likely to find their
original males on territories. That mean pair duration over the study (1.4 + 0.7 yr)
was approximately the same as mean tenure on territories by females suggests that
pair duration was limited by territory tenure of females.
Habitat Selection
Most male Flammulated Owls occupied a single territory their entire known
reproductive lives; only one breeding male changed territories in 62 male-years.
Moreover, males often continued to occupy original territories despite being unpaired
up to four consecutive years. Recall that mean tenure for males was calculated for
males that nested at least once. These males occupied a mean 58% (+ 3%) of all
territories annually (Chapter 2). Because I rarely captured unpaired males, I was
unable to determine their tenure on territories or extent to which they may have
dispersed. However, observation of several unpaired males, distinguished by unique
vocal characteristics (e.g., song pitch), indicated that most reoccupied their original
territories following return from migration (pers. observ.). An exception was an
97
unpaired male that occupied three different territories over four breeding seasons
(pers. observ.). Given that females (new and returning) only occupied half of all
territories annually and that breeding generally occurred on the same territories each
year (Chapter 5), many newly arriving males may settle on, and commit their
reproductive lives to, territories where breeding occurs irregularly. Consequently,
high site fidelity by males may counter predictions based on habitat selection models
that assume animals select habitats conferring highest reproductive success, and if
higher-quality habitats become available individuals should move to the new sites
(Fretwell and Lucas 1969). Thus, territory fidelity by Flammulated Owls may be
considered a suboptimal form of habitat selection with respect to territory quality
(Switzer 1993). It may be more profitable for a male of a long-lived species such as
the Flammulated Owl to await the probable arrival of a female in a territory where he
is familiar with the location of important nesting resources (see sources cited in
Introduction; Chapter 5), and possibly engage in extra-pair copulations when mates
are unavailable (Reynolds and Linkhart 1990). Males of other species including
raptors have been documented remaining on a single territory, even when higher-
quality territories apparently were available (Krebs 1971, Best 1977, Searcy 1979,
Bedard and LaPointe 1984, Janes 1984, Woolfenden and Fitzpatrick 1984, Lanyon
and Thompson 1986, Korpimaki 1988).
98
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CHAPTER V
DETERMINING HABITAT QUALITY FROM LONG-TERM DEMOGRAPHICS IN
BREEDING FLAMMULATED OWLS
Abstract. – Basing the determination of habitat quality on long-term demography is
generally regarded as a valuable approach for understanding how animal populations
use space, but it has little empirical support. From 1981-1999, I measured
demographic performance of Flammulated Owls (Otus flammeolus) in a 511 ha study
area of mixed conifer forest in central Colorado. Boundaries of 12 owl territories
remained generally stable over the study, enabling me to determine demographic
performance for territories rather than for individuals. I used demographic
parameters that distinguished among territories to infer relative territory quality so
that habitat conditions could be compared across territories and with non-territory
habitat. Territories differed in total breeding yr, because some territories usually were
occupied by breeding pairs annually while other territories usually were occupied by
bachelor males. Productivity varied among territories, ranging from 0 to 35 owlets.
Mean territory tenure, which I used as an estimate of survival, and pair duration did
not differ among territories. Breeding dispersal resulted in females moving to
territories where productivity was significantly higher.
Productivity was positively correlated with territory area in ponderosa
pine/Douglas-fir forests, and with greater crown volume in the second-largest (33.0 –
48.2 cm) of four tree dbh (diameter at breast height) categories. Productivity was not
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correlated with density of cavity-trees. However, cavity trees clearly distinguished
territory from non-territory habitat, given that non-territory habitat contained < 10%
of mean cavity-tree density within territories. Few structural characteristics
distinguished combined territories from non-territory habitat. In comparisons of non-
territory habitat with three classes of territory distinguished by differing productivity,
only the high-productivity class (consisting of just one territory) contained greater
tree density and basal area in the larger dbh categories. Based on the fact that
moderate-productivity territories showed no differences in forest structure from
unoccupied habitat, and that low-productivity territories (usually occupied by
bachelor males) actually contained denser forests and smaller trees than non-territory
habitat, at least some portions of non-territory habitat may have been suitable for
territory establishment except for scarcity of cavity trees. Monitoring of predation on
artificial nests for 1 yr and relative prey abundance for 2 yr revealed no patterns
among selected territories differing in productivity, suggesting these factors were not
associated with habitat quality.
My results indicate that cavity-tree availability primarily determined where
owls established territories, while forest type and structure determined whether a
territory was more often occupied by breeding pairs or by bachelor males. High-
quality breeding habitat for Flammulated Owls in this study was characterized as
mature, relatively open stands of ponderosa pine/Douglas-fir that contained sufficient
cavity trees for nesting.
Habitat correlations with bachelor yr differed markedly from correlations with
productivity, indicating that inferring habitat quality based on abundance or duration
107
of territory occupancy by males would be misleading. Breeding year may be a good
surrogate for productivity in future efforts to identify important breeding habitats for
this species, at least where other demographic parameters such as nesting success are
similar to mine.
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INTRODUCTION
Birds often occupy a range of habitats differing in resources necessary for
reproduction and survival (Fretwell and Lucas 1969, Cody 1985). These resources
include food, foraging sites, nesting sites, and places to avoid predators or
competitors (Steele 1993). Models predict that, in stable environments, birds settle
into high-quality habitats first and then progressively settle into lower-quality habitats
as high-productivity habitats are filled (Fretwell and Lucas 1969, Alatalo et al. 1985,
Pulliam 1988). This settlement pattern has been generally supported by field studies
(e.g., Brooke 1979, Zimmerman 1982, Moller 1983, Korpimaki 1988, Matthysen
1990).
Stable breeding populations depend on high-quality habitats because these
habitats confer sufficient reproduction and survival for populations to grow (Wiens
1986, Pulliam 1988). High-quality habitats also serve as sources of individuals
emigrating to low-quality or sink habitats, where reproduction is insufficient to
balance local mortality (Pulliam 1988). The distinction between high-quality and
low-quality habitats is important in conservation efforts because stable breeding
populations depend on high-quality habitats (Wiens 1986, Pulliam 1988, Howe et al.
1991). However, determining quality of breeding habitats has proven difficult and
procedures deemed best for distinguishing high-quality and low-quality habitats have
been little tested.
Most studies inferring the quality of avian breeding habitats have been based
on relative abundance (e.g., Anderson and Shugart 1974, Whitmore 1977, James and
Warner 1982, Wenny et al. 1993). This approach assumes that higher population
109
densities reflect breeding habitats of higher quality. However, identification of
important breeding habitats based on species abundance is not always reliable and
may misrepresent suitability of habitats for breeding (Van Horne 1983, Maurer 1986,
Martin 1992). High population densities may occur in sink habitats when prey
populations are cyclically high (Korpimaki 1988), or when surplus individuals
emigrate from source habitats (Krebs 1971). That population density is not always
correlated with reproductive success was shown in studies where dense populations
actually produced fewer offspring than less dense populations (Korpimaki 1988,
Vickery et al. 1992, Porneluzi et al. 1993). Studies have shown that in some habitats,
particularly those that may be sub-optimal, the majority of singing males remain
unmated through the breeding season (Probst and Hayes 1987, Gibbs and Faaborg
1990, Linkhart and Reynolds 1997, Burke and Nol 1998).
Reliable identification of high-quality habitats requires measures of fitness
(direct or indirect quantification of survival and reproductive success) because choice
of habitat features that increase individual fitness should be favored over evolutionary
time (Fretwell and Lucas 1969, Van Horne 1983, Martin 1992). However, relatively
few studies have evaluated survival or reproductive success in inferring habitat
quality because quantifying demographic variables requires considerable time and
labor (Wiens 1973). In addition, measures of survival are complicated by
immigration and emigration of individuals among breeding sites (Raphael et al.
1996). Some studies have shown that indirect measures of fitness may act as
surrogates for direct measures, including nesting behaviors (Vickery et al. 1992),
110
mate and site fidelity (Newton and Marquiss 1982, Haig and Oring 1988, Part 1994),
and breeding dispersal (Beletsky and Orians 1987, Matthysen 1990).
Long time periods are necessary to document patterns in demographic
variables of animals, as shown by studies of mammals (Peterson et al. 1984), birds
(Woolfenden and Fitzpatrick 1984), and reptiles (Gill et al. 1983). Short-term
measures of avian reproductive success may be biased by individual characteristics
such as age, condition, and experience (Coulson 1966, Nol and Smith 1987,
Korpimaki 1988, Ens et al. 1992). Stochastic environmental variation can also
compromise the reliability of short-term studies because these studies may fail to
capture the ecological and evolutionary consequences imposed by severe
environmental crunches (Grant 1986, Wiens 1986). Consequently, reliable inferences
of habitat quality require demographic data collected over long time periods (Van
Horne et al. 1997).
Choice of spatial scale is also important in investigations of habitat
relationships (Wiens 1986, Forman and Godron 1989, Kotliar and Wiens 1990).
While landscape-level studies are important for correlating avian population changes
with habitat (e.g., Martin 1981, Roth and Johnson 1993, Forsman et al. 1996),
territory-level studies may elicit the greatest understanding of habitat quality because
the quality and quantity of territory resources directly affect individual fitness
(Fretwell and Lucas 1969, Raphael et al. 1996). Identification of specific habitat
variables (e.g., nest-tree and foraging site characteristics; Matsuoka et al. 1997) in
territory-level studies also facilitate the development of species-specific management
plans.
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Few studies have linked demographic performance of individuals with
specific habitat features at the scale of individual territories. Most territory-level
studies have compared demographic performance with general habitat features, such
as vegetation types (Newton and Marquiss 1976, Fischer 1980, Zimmerman 1982,
Korpimaki 1988, Ens et al. 1992, Riddington and Gosler 1995). Only a few studies
(Janes 1984, Vickery et al. 1992, Breininger et al. 1995, Braden et al. 1997, Ortega
and Capen 1999) have statistically correlated demographic performance with specific
habitat variables such as perch sites or percentage of understory vegetation, and only
Janes (1984) reported demographic data collected from more than three yr. Thus,
little is known regarding the utility of assessing habitat quality by quantifying the
relationship between specific habitat characteristics on territories and long-term
demographic performance.
Here I identify the determinants of habitat quality for a breeding population of
Flammulated Owls by comparing habitat conditions in territories of varying quality as
inferred from demographic variables for a population in central Colorado studied
from 1981-1999. The owl is an obligate cavity-nester associated with montane
forests from the Rocky Mountains to the Pacific Coast, and from southern British
Columbia to Vera Cruz, Mexico (McCallum 1994a, Linkhart et al. 1998).
Flammulated Owls are insectivorous, feeding mostly on small lepidopterans
(Reynolds and Linkhart 1987), and are migratory, probably wintering in montane
forests of El Salvador, Guatemala, and Jalisco, Mexico (American Ornithologists’
Union 1983). The owls are listed as sensitive species in four western regions of the
USDA Forest Service (Verner 1994), because they are Neotropical migrants and are
112
associated with mature forests (McCallum 1994a, Linkhart et al. 1998), and because
densities have declined following timber harvests (Marshall 1957, 1988, Phillips et al.
1964, Franzreb and Ohmart 1978).
My objectives were: (1) to describe demographic performance (territory
occupancy, reproductive success, territory tenure, pair duration, and breeding
dispersal) on territories, and identify variables that distinguished among territories;
and (2) to identify components of habitat quality by correlating habitat variables with
demographic performance on territories. I evaluated habitat quality based on two
questions: (a) Across territories, was demographic performance associated with
forest type and structure (e.g., tree density, basal area, and crown volume)? (b) Does
forest structure differ among territories and between territory and non-territory (i.e.,
unoccupied) habitat? I predicted that reproductive success would be positively
associated with area in ponderosa pine/Douglas-fir, a forest type most frequently used
by foraging males in an earlier study (Linkhart et al. 1998), and I also predicted that
highest-quality territories would contain the lowest density of trees and trees with
largest mean diameter, while non-territory habitat would be characterized by the
reverse. I also evaluated three possible limiting factors associated with the owl’s
habitat relationships, and predicted that highest-quality territories were characterized
by highest densities of cavity trees, lowest rates of nest predation, and greatest prey
abundance.
METHODS
Study Area
113
The study was conducted on the Manitou Experimental Forest in Teller Co.,
Colorado. I established boundaries of the 511 ha study area after initial surveys
(1980) for territorial Flammulated Owls. After I confirmed the presence of owls,
boundaries were drawn around an area large enough to contain approximately 20
territorial males, based on an estimate of territory size for this species (274 m in
diameter; Marshall 1939). Forests within the study area generally consist of (1)
ponderosa pine (Pinus ponderosa) mixed with Douglas-fir (Pseudotsuga menziesii),
generally on ridgetops and south- and west-facing slopes, (2) quaking aspen (Populus
tremuloides) stands on lower slopes and bottoms of moist drainages, (3) quaking
aspen stands mixed with blue spruce (Picea pungens) in bottoms, lower slopes, and
benches in moist areas, and (4) Douglas-fir mixed with blue spruce, on higher slopes
in drainages and on north-facing slopes. Tree cutting on the study area has not
occurred since the 1880s, when a light harvest for railroad ties occurred. Snags and
trees with cavities were relatively abundant throughout the study area (Reynolds et al.
1985). The forest understory, consisting of over 100 species of grasses, forbs, and
shrubs, was poorly developed in all but the moist creek bottoms (Reynolds et al.
1985). Terrain was moderately steep (20-80% slope) and elevations ranged from
2,550-2,855 m. The study area is surrounded by forests composed of a similar mix of
forest types and ages.
Delineating Territories and Locating Nests
Each spring and summer from 1981-1999, I searched the entire study area for
territorial males (Reynolds and Linkhart 1984). I identified territory boundaries by
marking territorial song-trees of males (Reynolds and Linkhart 1984) and using radio-
114
telemetry in 1982-1983, which assisted in determining range in territory sizes and
identifying topographic characteristics of boundaries (Linkhart et al. 1998). Most
territory boundaries changed little from year to year (Fig. 18), despite male turnover
on each territory (see Results), as evidenced by the fact that males commonly sang
from the same groups of trees (and often the same trees) throughout their territories
annually, particularly along boundaries that were in close proximity to those of
neighboring males (Linkhart 1984, Linkhart et al. 1998). While I may have missed
some annual fluctuation along boundaries less defended by males due to absence of
adjacent neighbors, lack of annual change in most boundaries indicated that territories
were generally constant in time and space.
Each year I located all suitable nesting cavities (tree cavities with entrance
diameters > 4 cm, generally excavated by Northern Flickers [Colaptes auraetus] and
occasionally sapsuckers [Sphyrapicus spp.]) within territories and checked each for
nesting owls (Reynolds and Linkhart 1984). Unpaired males typically sang
throughout a breeding season, whereas singing in paired, nesting males dramatically
declined after egg-hatch (Reynolds and Linkhart 1987b). Due to time and effort spent
searching for owls and nests each year, I was confident that all nests were located. I
found most nests during incubation (late May and early June) and checked the status
of nests at least weekly (often two or three times per week) until the young fledged
(mid July). Breeding adults were captured at nests (occasionally on perches or day
roosts) and banded with U. S. Fish and Wildlife Service leg bands (Reynolds and
Linkhart 1984). Sex of adults was determined by behavioral and morphological
characteristics (see Reynolds and Linkhart 1984). I banded owlets when they were
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Figure 18. Location of owl territories (black polygons) on the Manitou Experimental
Forest study area (white boundary), 1981-1999. Heavy white polygons
represent territories (A15 and A24) not occupied after 1984 and light white
polygons represent territories (A4, A8, A11, and A29) prior to boundary shifts.
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14-21 d old (fledging occurs at 22-24 d; Reynolds and Linkhart 1987b). A total 215
owls were banded over the study: 146 owlets (sex not determined) and 69 adults (29
males and 40 females). Because unpaired males were difficult to capture, all banded
males but one were owls that had > 1 nesting attempt.
Demographic Performance
Because territories generally were constant in time and space over the study,
despite individual turnover on each territory, I calculated cumulative demographic
performance over 19 yr for 12 territories. I omitted 2 territories (A15 and A24) from
calculations of demographic performance because they were not occupied after 1984.
For each territory I calculated the following variables: (1) occupied yr, the sum of all
yr (out of 19 yr possible; not necessarily consecutive) territories were occupied by
breeding pairs or bachelor males; (2) breeding yr, the total yr (not necessarily
consecutive) territories were occupied by breeding pairs; (4) bachelor yr, the total yr
(not necessarily consecutive) territories were occupied by unpaired males; (5) owlets,
the total number of banding-age owlets produced by pairs over the study; and (6)
nesting success, calculated by dividing the number of nests over the study that
fledged at least one young by the total number of nests in which eggs were layed. I
excluded 6 nests from calculations of nesting success whose outcome was uncertain
(A12 territory-1981; A18-1981; and A10-1990) or failed due to anthropogenic causes
(A18-1984, A29-1991, and A29-1994). In addition, fidelity and dispersal variables
were quantified for individual territories based on annual capture-recapture data on
adults banded over the study, including: (7) mean pair duration—the mean total yr in
which the same male and female remained paired, calculated by dividing the summed
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duration of all pair bonds by the total number of new pair bonds; and (8) mean
territory tenure—the mean yr that individuals of each sex remained on the same
territory, calculated by dividing total yr of occupancy by banded individuals on a
territory by total number of uniquely banded individuals on the territory. I used
territory tenure as an estimate of survival. Although I did not know the fate of owls
that failed to return to the study area, I assumed they died because movement between
territories (breeding dispersal) only occurred infrequently. Finally, I quantified (9)
breeding dispersal by summing the frequency of dispersal events on each territory.
In calculating fidelity variables, I presumed a male banded on a territory in
year X was present on the same territory in year X + 1, despite not recapturing him in
year X + 1, if: (1) a male was heard singing on the territory in year X + 1, and (2)
the same banded male was recaptured in a subsequent year on the same territory.
This assumption was based on the fact that I did not document any instances in which
a male left a territory and was recaptured in a subsequent year on the same territory. I
presumed a banded female that nested in year X did not return to her same original
territory in year X + 1 if no nest was found, since I did not document any instances in
which a female failed to nest in one year but nested on the same territory in the
previous and subsequent years.
My estimates of territory tenure and pair duration included adults banded in
1981 (n = 9 unknown histories) and adults banded and/or recaptured in 1999 (n = 10
unknown futures), because estimates of these variables for “fringe” individuals did
not significantly differ from “inclusive” individuals whose breeding lifespans began
after 1981 and terminated before 1999. Mean (+ standard error, SE) total yr of return
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for fringe males (n = 4 in 1981 and 5 in 1999) was 3.9 + 1.3 yr, and mean total yr of
return for inclusive males was 2.6 + 0.5 yr (n = 22; t = -0.94, df = 11, P = 0.37, two-
tailed). Likewise, mean total yr of return for fringe females (n = 5 in 1981 and 5 in
1999) was 2.4 + 0.6 yr and mean total yr of return for inclusive females was 1.7 + 0.3
yr (n = 31; t = -1.03, df = 14, P = 0.32, two-tailed). Mean pair duration for fringe
pairs (n = 5 in 1981 and 5 in 1999) was 1.6 + 0.2 yr and pair duration for inclusive
pairs was 1.4 + 0.1 SE yr (n = 44; t = -0.72, df = 12, P = 0.49, two-tailed).
I used Chi-square goodness-of-fit tests to determine if specific demographic
variables differed among the 12 territories, where the expected value in each case was
the variable mean across all territories. Degrees of freedom for these tests was 10
instead of 11 to account for estimation of the expected value equal to the variable
mean (Daniel 1990). I then used demographic variables that distinguished among
territories in correlations with habitat variables. In addition, I used a paired t-test
assuming unequal variances to compare productivity (total owlets over 19 yr) on
original territories where females nested before dispersing to productivity on new
territories where females nested after dispersing. For each female, I paired
productivity on the original territory with productivity on the new territory.
Vegetation Sampling
During summer 1998 I established sampling points 100 m apart on 11 east-
west transects set at 200 m intervals across the study area. To increase overall sample
sizes within territories and sample sizes among forest types of territories, I
supplemented the 1998 transects with additional 100 m interval transects within
territories in 1999. Overall, during 1998-1999 I established 209 territorial sampling
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points (18 + 2 territory-1) and 143 non-territorial sampling points. Locations of
sampling points were determined with Global Positioning System (GPS; Trimble),
and mapped in Arcview (ESRI 1995).
I quantified forest overstory (any tree > 2 m tall and > 2.5 cm dbh, diameter at
breast height) using an angle-gauge (Dilworth and Bell 1977), and identified 6-10
sample trees (mean 7.6 + 0.1) in a variable-radius circle around each sampling point.
For each sample tree I determined species, dbh, and crown volume (volume of live
foliage, based on shape, width, and height of the crown; sensu Mawson et al. 1976).
Based on all sample trees, at each sampling point I calculated estimates of crown
volume ha-1 (CRVL), basal area ha-1 (BA), and trees ha-1 (TPH; Dilworth and Bell
1977). I also calculated estimates of these variables according to 4 dbh size-
categories (2.5-17.7 cm, 17.8-32.9 cm, 33.0-48.2 cm, and > 48.3 cm; Table 1), based
on distribution of tree sizes across the study area.
I quantified forest understory (herbs, shrubs, and trees < 2 m tall and 2.5 cm
dbh) at sampling points using line-intercept (Canfield 1941) along two perpendicular
12.5 m tape lines (following cardinal directions) which crossed on the sampling point.
At 0.5 m intervals I recorded all species that ‘hit’ a 6 mm diameter vertical rod within
four height categories: 1-4 cm; 5-49 cm; 50-199 cm; and > 200 cm. I calculated
proportion of cover in each height category by dividing number of hits by total
number of sampling intervals (50; Table 4).
Determination of Forest Types
I used classification and regression tree analyses (CART; Breiman et al. 1984)
to develop a model of forest types on the study area based on data associated with the
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Table 4. Forest overstory and understory variables used in correlations with
demographic variables.
Variable Description
MN_DBH mean dbh (cm) of individual trees
BA basal area (m2 ha-1)
T_BAD1 basal area (m2 ha-1) in dbh category 1 (2.5-17.7 cm)
T_BAD2 basal area (m2 ha-1) in dbh category 2 (17.8-32.9 cm)
T_BAD3 basal area (m2 ha-1) in dbh category 3 (33.0-48.2 cm)
T_BAD4 basal area (m2 ha-1) in dbh category 4 (> 48.3 cm)
P_BAD1 proportion of BA in dbh category 1 (2.5-17.7 cm)
P_BAD2 proportion of BA in dbh category 2 (17.8-32.9 cm)
P_BAD3 proportion of BA in dbh category 3 (33.0-48.2 cm)
P_BAD4 proportion of BA in dbh category 4 (> 48.3 cm)
TPH trees ha-1
T_TPHD1 trees ha-1 in dbh category 1 (2.5-17.7 cm)
T_TPHD2 trees ha-1 in dbh category 2 (17.8-32.9 cm)
T_TPHD3 trees ha-1 in dbh category 3 (33.0-48.2 cm)
T_TPHD4 trees ha-1 in dbh category 4 (> 48.3 cm)
P_TPHD1 proportion of TPH in dbh category 1 (2.5-17.7 cm)
P_TPHD2 proportion of TPH in dbh category 2 (17.8-32.9 cm)
P_TPHD3 proportion of TPH in dbh category 3 (33.0-48.2 cm)
P_TPHD4 proportion of TPH in dbh category 4 (> 48.3 cm)
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MN_CRVL mean crown volume (m3) of individual trees
CRVL total crown volume (m3 ha-1)
T_CRVLD1 crown volume (m3 ha-1) of trees in dbh category 1 (2.5-17.7 cm)
T_CRVLD2 crown volume (m3 ha -1) of trees in dbh category 2 (17.8-32.9 cm)
T_CRVLD3 crown volume (m3 ha-1) of trees in dbh category 3 (33.0-48.2 cm)
T_CRVLD4 crown volume (m3 ha-1) of trees in dbh category 4 (> 48.3 cm)
P_CRVLD1 proportion of CRVL in dbh category 1 (2.5-17.7 cm)
P_CRVLD2 proportion of CRVL in dbh category 2 (17.8-32.9 cm)
P_CRVLD3 proportion of CRVL in dbh category 3 (33.0-48.2 cm)
P_CRVLD4 proportion of CRVL in dbh category 4 (> 48.3 cm)
UNVG_5 proportion of understory cover in height category 1-4 cm
UNVG5_50 proportion of understory cover in height category 5-49 cm
UNVG_50 proportion of understory cover in height category 50-199 cm
UNVG_200 proportion of understory cover in height category > 200 cm
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352 sampling points: field-identification of forest types; color-band reflectance
derived from thematic mapper (LANDSAT) satellite image (30 m resolution; data
gathered 14 September 1997) in GIS; and slope, aspect, and elevation derived from a
digital elevation model (DEM; 30 m resolution) in GIS. CART analyses resulted in
the delineation of six forest types on the study area: (1) ponderosa pine/Douglas-fir;
(2) Douglas-fir; (3) Douglas-fir/blue spruce; (4) quaking aspen; (5) quaking
aspen/blue spruce; and (6) Douglas-fir/limber pine (Figure 3). Overall model
accuracy was 83%, based on comparing field-assigned and analysis-assigned forest
types at sampling points. I then used ArcView (ESRI 1995) to map the six forest
types and determine area (ha) and percentage of area in each forest type for the study
area, and area (ha) and percentage of area of forest types in each territory.
Comparison of Structure Among Forest Types
To compare forest structure (overstory and understory variables; Table 4)
among disproportionately sampled forest types, I weighted individual sampling points
proportional to amount of area in each forest type and then pooled these weighted
values within each forest type and calculated from these a mean forest-type value for
each habitat variable. This is similar to estimation for stratified random sampling
(Cochran 1977), with the overall sample size maintained at the total number of
sampling points:
Wx= (a1/at)nt / n1 (Equation 1)
where, Wx was the weight factor for sampling points in forest type x, a1 was area
occupied by forest type x, at was the total area occupied by all forest types, nt was the
total number of sampling points, and n1 was the number of sampling points in forest
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type x. I then used ANOVA and Tukey’s Test (PROC GLM; SAS 1996) to compare
specific forest structure variables (Table 1) among the six forest types.
Comparisons Between Vegetation And Owl Demography
Determination of territory habitat was based on all sampling points within
combined territories. Boundaries of most territories remained fixed over the study
(Fig. 18). For four territories (A4, A8, A11, and A29) whose boundaries shifted
during 1983-1987, I evaluated habitat variables based on boundaries after the shifts
occurred, since new boundaries remained unchanged for the remainder of the study
and accounted for the majority (63-84%) of total study yr. I omitted two territories
(A15 and A24) from analyses of habitat quality because these territories were not
occupied after 1984 and much of the area within their boundaries was incorporated
into adjacent territories. I based determination of non-territory habitat on all
sampling points located outside of territory boundaries.
Relationship between forest type and structure and owl demography.—To assess the
relationship between forest type and demographic performance across territories, I
determined the area (ha) and proportion of total area of forest types within owl
territories using ArcView (ESRI 1996). I used Pearson product correlation analyses
(PROC REG; SAS 1996) to examine correlations among forest types and
demographic performance.
To evaluate relationships between forest structure (forest and understory
variables; Table 1) and demographic performance across territories, I weighted
individual sampling points proportional to amount of area in each forest type within a
particular territory using Equation 1. I then pooled these weighted values within each
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territory and calculated from these a mean territory value for each habitat variable. I
used Pearson product correlation analyses (PROC REG; SAS 1996) to examine
correlations between forest structure variables and demographic variables. I then
used stepwise regression analyses (PROC REG; SAS 1996) to evaluate the inter-
correlations among forest structure variables found significantly correlated with
demographic variables, and to address the increased probability of incurring Type 1
errors associated with multiple univariate comparisons.
Comparison of territory and non-territory habitat.—I compared forest structure in
territory vs non-territory habitat in two ways. First, I assessed whether forest
structure for combined territories differed from non-territory habitat. I weighted
sampling points to account for under- or over-sampling of each forest type within
combined territories or non-territory habitat using Equation 1. I used ANOVA
(PROC GLM; SAS 1996) to compare territory vs non-territory habitat for specific
variables (Table 4).
To further determine if forest structure differed between territory and non-
territory habitat, I compared non-territory habitat to three classes of territories, based
on the range in total owlets over the study (see Results): high-productivity (> 23
owlets), moderate-productivity (13 - 23 owlets), and low-productivity (< 8 owlets). I
defined these classes based on natural "breaks" in the range of total owlets; 12 owlets
separated high-productivity and moderate-productivity habitat classes, and 5 owlets
separated the moderate-productivity and low-productivity habitat classes. Although
this approach resulted in the high-productivity class containing only one territory
(A4) compared to the moderate-productivity (5 territories) and low-productivity (6
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territories) classes, I evaluated this territory separately because preliminary analyses
indicated its forest structure differed significantly from all other territories.
To compare forest structure in the three classes of owl habitat with non-
territory habitat, within each habitat class I weighted sampling points to account for
under- or over-sampling of each forest type using Equation 1. I used ANOVA and
Tukey’s Test (PROC GLM; SAS 1996) to compare specific forest structure variables
(Table 4) between territory and non-territory habitat.
Possible Limiting Factors Associated With Habitat Relationships
Density of Cavity Trees.—I annually located all snags (standing, dead trees) and live
trees with suitable cavities for nesting (hereafter, cavity trees) within territories.
Beginning in 1983, as part of a long-term investigation to determine the spatial and
temporal dynamics of cavity trees (Reynolds et al. 1985), I also mapped the location
of cavity trees outside of territories.
Quaking aspen cavity-trees, which accounted for the majority (> 80%) of owl
nests annually (unpubl. data), typically had shorter longevity (snags 1-10 yr, live trees
5-15 yr) than conifer cavity-trees (snags > 10 yr, live trees > 20 yr; pers. observ.).
However, the general location (i.e., forest patch) and relative density of deciduous
cavity-trees changed little from 1981-1999. Given the observed constancy in spatial
dynamics and density of conifer and deciduous cavity-trees over time, I used the
location of cavity trees in 1999 as the basis for inferring general location and density
of cavity trees over the19 yr study. I mapped the locations of all cavity trees found in
1999 on a DEM layer in ArcView (ESRI 1995), and determined the density of cavity
trees within territories and outside territories. I assessed correlation between density
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of cavity trees and demographic performance on territories with Pearson product
correlation analysis (PROC REG; SAS 1996).
Nest Predation.—Effects of nest predation were determined in two ways. First, I
monitored predation of artificial nests in six owl territories during summer 1999: two
territories producing the most owlets over the 19 yr study, two territories producing
the least owlets, and two territories producing an intermediate number of owlets. Red
Squirrels (Tamiasciurus hudsonicus) were inferred as the primary predator of owl
nests because I observed no other avian or mammalian predators of tree cavities over
the study (but see Linkhart and Reynolds 1994). I determined sites for six artificial
nests in each territory by placing two nests in each of the three most common forest
types: ponderosa pine/Douglas-fir, Douglas-fir, and quaking aspen/blue spruce.
Where possible, I used unoccupied, natural tree cavities with entrance diameters > 4
cm. In territories lacking sufficient natural cavities, I attached wooden nest boxes (40
x 20 x 20 cm, entrance 6 cm in diameter) to live trees 3-5 m above ground. Boxes
had a southern exposure, which approximated the orientation of most owl nests
(unpubl. data), and contained 1-3 cm of partially decomposed woody debris. Of 36
total artificial nests, 18 were in nest boxes. Most territories contained 3 nest boxes, 2
of which were generally in Douglas-fir forests because these forests contained the
fewest natural cavities. I placed two fresh Bobwhite Quail (Colinus virginianus) eggs
in each artificial nest beginning 10 June, when owl eggs were being incubated, and
monitored weekly predation rates until 25 July, when all but one owl brood had
fledged. Eggs lost to predation were replaced weekly, as well as eggs that remained
in nests beyond two weeks. Differences in predation rates of artificial nests among
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territories and among forest types were determined with chi-square (PROC GLM),
and correlations between predation rates and productivity on territories were
determined with Pearson product correlation analysis (PROC REG; SAS 1996).
Second, during summer 1999 I estimated relative density of Red Squirrels in
the six owl territories where I monitored predation of artificial nests (see above).
Since Red Squirrel territories typically contain one large midden, which contain
caches of conifer cones for winter food supplies (Gurnell 1984, Hurly and Lourie
1997), I used density of large middens as a measure of relative squirrel density in owl
territories. I mapped all large middens (dimensions > 2 x 3 m, depth > 20 cm) on a
DEM in Arcview (ESRI 1995). I assessed differences in midden density among
territories with ANOVA, and differences among forest types with Tukey’s Tests
(PROC GLM; SAS 1996). Correlations between midden density and nest-predation
rates, and between midden density and demographic performance, were determined
with Pearson product correlation analysis (PROC REG; SAS 1996).
Relative Prey Abundance.—I used black-light traps (Southwood 1981) to estimate
relative arthropod abundance in two owl territories, one producing the most owlets
(A4) and one producing the fewest owlets (A18) over the study. One black-light trap
was placed in the interior of the largest available forest patches of ponderosa
pine/Douglas-fir, Douglas-fir, and quaking aspen/blue spruce. Black-light traps were
placed between adjacent trees (3-4 m apart) and hung 1.5 m above ground adjacent to
a vertical white cloth sheet (1.5 x 2.0 m). On sampling nights (see below), arthropods
that landed on the sheet were counted every quarter-hour from 2100-2300 hr and
averaged over the 2 hr. I counted all flying arthropods, mostly lepidopterans, whose
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length from base of antennae to tip of wing was 15-29 mm. Lepidopterans of this
size were the primary prey delivered by males to nests (Linkhart et al. 1998, unpubl.
data).
I sampled arthropod populations in 1998 and 1999 during two owl nesting
stages: incubation (18 May-11 June), and nestling (23 June-23 July). I compared
arthropod abundance between the two territories by simultaneously sampling the
same forest type (e.g., ponderosa pine/Douglas-fir) in both territories. Three
sampling nights were required to make all three intra-forest type comparisons
between territories. I compared the 2 territories during 4 trap-nights (i.e., 4 nights in
each of the forest types) of the incubation stage (2 nights each in 1998 and 1999), and
5 trap-nights of the nestling stage (3 nights in 1998 and 2 nights in 1999).
Differences in relative prey abundance between territories were determined with
ANOVA (PROC GLM; SAS 1996). For these and all other statistical analyses, I
determined whether distributions of variables deviated significantly from normality
(PROC UNIVARIATE; SAS 1996), and if necessary performed data transformations
(log, square root, and arcsin) to achieve normality and reran the procedure. I use a
significance level of P = 0.05 and present means + standard error (SE).
RESULTS
Temporal/Spatial Constancy of Territories
I monitored occupancy on 14 owl territories from 1981-1999 (Fig. 1).
Territories were generally constant in time and space despite individual turnover on
each territory, with a few exceptions. First, the A24 territory was only occupied from
1981-1983. In 1984, a new male in an adjacent territory (A29) expanded the
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boundaries of his territory to include much of the western portion of A24 territory,
and these new boundaries did not change over the remainder of the study (Fig. 18).
Second, the male in A4 territory expanded the boundaries of his territory in 1983 to
include much of the eastern portion of A15 territory, which was only occupied in
1981-1982 and 1984 (Linkhart et al. 1998; Fig. 18). I did not know the boundaries of
either territory in 1984, when only A15 territory was occupied by a breeding pair, but
for the remainder of the study A4 territory contained the eastern half of A15 territory.
Finally, in 1988 the male in A8 territory expanded his territory north into the southern
portion of A11 territory, after the male in A11 territory apparently did not return from
migration, and northeast into the northern portion of A15 territory (Fig. 18). After
1988, A11 territory contained only the northern portion of the original A11 territory
(Fig. 18). In each of the above instances, shifts in territory boundaries occurred when
> 1 males in adjacent territories did not return from migration. Boundaries of all
other territories remained unchanged over the study. Males generally returned to
territories annually; territory fidelity was 98% and annual turnover was 10% (Chapter
4). Consequently, newly arriving males in the spring typically filled geographic voids
left by predecessors. The overall high stability of territory boundaries allowed me to
compare their habitat characteristics to their demographic performance over the entire
study. Below I report demographic performance for the 12 territories (omitting A15
and A24) active over the 19 yr.
Demographic Performance on Territories Territory Occupancy.—Territories differed in total yr (occupied yr) they were
occupied by owls, ranging from 3-19 yr (χ2 = 17.3, df = 10, P = 0.07; Table 5).
Table 5. Demography on owl territories from 1981-1999.
Territory Occupancy Reproductive Success
Territory
Occupied yr
Bachelor yr Breeding yra
No. success. nestsb
Nest success
(%)b
Owlets Mean owlets brood-1
Mean owlets
yr-1
A4 19 3 16 15 93.8 35 2.2 1.8
A8 16 5 11 9 81.8 23 2.1 1.2
A29 16 4 12 7 70c 17 1.7 0.9
A10 13 4 9 5 62.5c 13 1.4 0.7
A11 14 6 8 7 87.5 17 2.1 0.9
A13 12 5 7 7 100 14 2.0 0.7
A12 14 10 4 3 100c 7 1.8 0.4
A27 9 7 2 2 100 5 2.5 0.3
A20 6 3 3 3 100 8 2.7 0.4
A7 9 6 3 1 33.3 3 1 0.2
A18 13 10 3 1 100c 2 1 0.1
A2 4 3 1 0 0 0 0 0
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MEAN 12.1 5.5 6.6 5.0 77.4 12.0 1.7 0.6
SE 1.3 0.7 1.4 1.2 9.2 2.9 0.2 0.1 a Identical to nesting attempts, since owls only attempted to breed a maximum of once yr-1 and did not renest if nests failed. b Nests that fledged at least one owlet c Excludes nesting attempts where outcome of nest was unknown or nest failed due to anthropogenic causes: A29-2 attempts; A10-1
attempt; A12-1 attempt; A18-2 attempts
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Territory tenure- Males Territory tenure-Females Pair bonding Territory
Total Males
Mean tenure (yr) Total Females
Mean tenure (yr)
Total unique pair bonds
Mean duration (yr)
A4 5 3.4 9 1.6 10 1.4
A8 4 2.8 4 2.8 7 1.4
A29 2 7.0 5 2.0 5 2.0
A10 3 3.7 5 1.4 5 1.4
A11 2 5.5 5 1.6 5 1.6
A13 4 2.3 6 1.2 6 1.2
A12 1 3.0 2 1.5 2 1.5
A27 1 6.0 2 1.0 2 1.0
A20 2 1.5 2 1.5 2 1.5
A7 1 1.0 3 1.0 1 1.0
A18 1 1.0 1 1.0 1 1.0
A2 1 1.0 1 1.0 1 1.0
MEAN 2.2 3.2 3.8 1.5 3.9 1.3
SE 0.4 0.6 0.7 0.1 0.8 0.1
Territories were occupied by breeding pairs (breeding yr) only 35% of total yr (6.6 +
1.1 yr; not necessarily consecutive), and breeding yr differed among territories (χ2 =
36.9, df = 10, P < 0.001; Table 5). Similarly, territories were occupied by bachelor
males (bachelor yr) 29% of total yr (5.5 + 0.6 yr; not necessarily consecutive), but
bachelor yr did not differ among territories (χ2 = 12.2, df = 10, P = 0.27; Table 5).
Some territories usually were occupied by breeding pairs annually while some
territories usually were occupied by bachelor males. Three territories (A4, A8, and
A29) had > 12 breeding yr and five territories (A12, A7, A18, A27, and A2) had more
bachelor yr than breeding yr (Table 5).
Reproductive Success.—Pairs on territories had a maximum of one nesting attempt
annually. Because I never observed renesting by owls, even when nests failed during
early incubation, mean nesting attempts territory-1 (6.1 + 1.0) over the study was
identical to mean breeding yr. Nesting success over the study was high; 82% of all
nests were successful (60 of 73 nests, excluding 6 nests whose outcome was uncertain
or failed due to anthropogenic causes). The 13 nest failures, which occurred in 8
different territories, were all attributed to nest predation. Four territories never had
nest failure and three territories had < 33% nest success, although all of these
territories except one (A13) had < 3 nest attempts (Table 5).
Total owlets territory-1 (12.0 + 2.9) differed among territories (χ2 = 93.3, df =
10, P < 0.001; Table 5). The most productive territory (A4; 35 owlets) produced 60%
more owlets than the next most productive territory (A8; 22 owlets), while the least
productive territory (A2) failed to produce any owlets (Table 5). Of 144 total owlets
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produced by all 12 territories over the study, 25% (3 territories) produced 52% of
owlets, and 50% (6 territories) produced 83% of owlets (Fig. 19).
Territories had a mean brood size of 1.7 + 0.2 owlets, including successful
(nests fledging > 1 owlet) and unsuccessful nests, and mean brood size did not differ
among territories (χ2 = 7.42, df = 10, P = 0.69; Table 5). Excepting one brood of 1
owlet and one brood of 4 owlets (both in A4 territory), all broods contained either 2
or 3 owlets. Overall, territories produced a mean 0.6 + 0.1 owlets yr-1 over the 19 yr
study (Table 5).
Pair Duration.—Mean pair duration territory-1 was 1.3 + 0.1 yr, and did not differ
among territories (χ2 = 0.8, df = 10, P = 0.99; Table 5). Only one divorce was
documented over the study; a female who had nested successfully once with her
original mate in A10 territory in 1997 was found nesting with a new male in A8
territory in 1999. Territories had a mean 3.9 + 0.8 unique pair bonds, and differences
among territories were attributable to breeding yr (Spearman’s p = 0.86, P < 0.001;
Table 5) since duration of most pair bonds was 1 yr (74%; Chapter 4).
Territory Tenure.—Mean territory tenure, which I used as an estimate of survival, for
males was 3.2 + 0.6 yr (Table 5), and did not differ among territories (χ2 = 14.16, d.f.
= 10, P = 0.17). Three territories (A27, A29, and A11) had mean tenure by males >
5.5 yr, and each of these was occupied by < 2 males. Mean territory tenure for
females was 1.5 + 0.1 yr (Table 5), and this also did not differ among territories (χ2 =
2.0, df = 10, P = 0.99). Mean longest tenure by females on a territory was 2.8 yr, and
this territory (A8) was occupied by 4 females (Table 5).
Breeding Dispersal.—Breeding dispersal by females occurred on eight occasions
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Figure 19. Contribution of individual territories to total owlets produced by
combined territories from 1981-1999.
0
20
40
60
80
100
0 20 40 60 80 100
Cumulative Percent of Territories
Cum
ulat
ive
perc
ent o
f ow
lets
137
over the study, and usually occurred when males on original territories did not return
after spring migration (Chapter 4). Four territories (A4, A8, A29, and A12) were
recipients of 2 dispersing females each, although one pair of owls also dispersed from
the latter territory including the female immigrant. Females dispersed to territories
having significantly higher productivity (paired t = -2.53, df = 7, P = 0.04; Table 6).
The only dispersing male also moved from a territory having lower productivity (A12
territory; 7 owlets) to a territory having higher productivity (A4 territory; 35 owlets).
Distribution and Structure of Forest Types
Forests of ponderosa pine/Douglas-fir and Douglas-fir accounted for the
greatest percentage of the study area (53% and 23%) while each of the other forest
types occupied < 10% (Table 7, Fig. 20). Forest composition of combined territories
was similar to non-territory habitat; each forest type differed by < 4% between the
two areas (Table 7). However, among territories, percentage of area in ponderosa
pine/Douglas-fir was greatest in A4 and least in A12, while the percentage of area in
Douglas-fir was greatest in A18 and least in A4 (Table 7). All other forest types
accounted for a mean < 10% of territories (Table 7).
Overstory structure of ponderosa pine/Douglas-fir forests differed
significantly from most other forest types (Table 8). Ponderosa pine/Douglas-fir
forests contained less basal area (BA), lower tree density (TPH), and larger mean
crown volume (MN_CRVL), than all other forest types except quaking aspen, and
contained trees with larger mean dbh (MN_DBH) than Douglas-fir, Douglas-fir/blue
spruce, and quaking aspen/blue spruce (Table 8). Compared to Douglas-fir forests in
the two smallest dbh categories (2.5-17.7 cm and 17.8-32.9 cm), ponderosa
138
Table 6. Comparison of owlets produced over 19 yr on territories where females
nested before dispersal (“original territory”) to owlets produced on territories to
which those same females dispersed (“new territory”).
Original territory New territory Net
difference in
Territory (yr)
Owlets Territory (yr) Owlets owlets (%)
A15 (1982) 7 A4 (1983) 35 400
A29 (1983) 17 A12 (1984) 7 -59
A24 (1983) 4 A29 (1984) 17 325
A15 (1984) 7 A12 (1985) 7 0
A12 (1986) 7 A4 (1987) 35 400
A7 (1990) 3 A29 (1991) 17 467
A13 (1996) 14 A8 (1998) 23 64
A10 (1997) 13 A8 (1999) 23 77
MEAN 9.0 MEAN 20.5 MEAN 209
SE 1.8 SE 3.8
139
Table 7. Area and percentage of forest types in owl territories, non-territory habitat,
and over the entire study area.
Pipo/Psme1 Psme2 Psme/Pipu3 Potr4 Potr/Pipu5 Psme/Pifl6
Terr7 ha % ha % ha % ha % ha % ha % Totalha)
A2 4.1 62 1.2 18 0.2 3 1.1 16 0 0 0.1 1 6.6
A4 18.7 88 1.4 6 0 0 0.6 3 0.4 2 0.3 1 21.3
A7 6.0 55 1.5 14 0 0 0 0 2.3 20 1.3 11 11.1
A8 14.0 79 2.8 16 0 0 0.3 2 0.5 3 0.2 1 17.6
A10 11.3 61 3.6 19 1.3 7 0 0 1.9 10 0.5 3 18.5
A11 6.1 49 3.2 25 0.5 4 0 0 1.5 12 1.3 10 12.5
A12 1.7 26 1.8 27 2.0 30 0.6 10 0.5 7 0 0 6.6
A13 7.9 38 5.3 26 5.3 26 0.1 <1 1.3 6 0.7 4 20.6
A18 5.9 33 8.3 47 1.8 10 0 0 1.1 6 0.8 5 17.8
A20 7.7 42 5.8 32 1.2 6 0 0 2.4 13 1.3 7 18.3
A27 1.9 35 1.5 28 0.4 7 0 0 1.6 30 0 0 5.4
A29 18.6 75 3.1 12 0 0 0 0 3.0 12 0.2 1 24.8
Mean 8.6 56 3.3 22 1.0 7 0.2 1 1.4 10 0.5 4 15.1
N 159 50 77 24 29 9 3 1 23 7 26 8 319
SA8 271 53 120 23 42 8 6 1 40 8 33 7 511
1 ponderosa pine/Douglas-fir; 2 Douglas-fir; 3 Douglas-fir/blue spruce; 4 quaking
aspen; 5 quaking aspen/blue spruce; 6 Douglas-fir/limber pine; 7 territory; 8 study
area
140
Figure 20. Distribution of owl territories, cavity trees, and forest types on the
Manitou Experimental Forest study area. A15 and A24 territories (‘temp
territ’; temporary territories) were not occupied after 1984, and A40 (black
ellipse) was an unstudied territory on the edge of the study area. Legend:
‘‘terr cavit’ = cavity trees within owl territories; ‘non-terr cavit’ = cavity trees
outside of owl territories; ‘p pine’ = ponderosa pine; ‘D-fir’ = Douglas-fir;
‘spruce’ = blue spruce; ‘aspen’ = quaking aspen; and ‘l pine’ = limber pine.
Table 8. Comparison of forest structure variables (mean + SE) among forest types. Different letters denote significant differences
among forest types (Tukey’s tests, df = 346, p = 0.05).
Forest Type
Variablea (units) Ponderosa
pine/Douglas-fir
Douglas-fir Douglas-
fir/blue spruce
Quaking aspen Quaking aspen
/blue spruce
Douglas-
fir/limber pine
MN_DBH (cm) 31.0 + 0.6 a 24.3 + 0.6 b 22.9 + 1.0 b 22.4 + 2.1 ab 23.5 + 1.0 b 27.7 + 2.2 ab
BA (m2 ha-1) 22.4 + 1.0 a 31.9 + 1.2 b 34.2 + 2.2 b 24.0 + 1.3 ab 31.8 + 1.9 b 31.8 + 4.5 b
T_BAD1 (m2 ha-1) 3.8 + 0.5 a 10.4 + 1.0 b 11.3 + 1.5 b 6.4 + 1.9 ab 9.9 + 1.5 b 6.3 + 1.7 ab
T_BAD2 (m2 ha-1) 9.8 + 0.7 a 14.9 + 0.8 b 17.6 + 1.7 b 13.4 + 2.6 ab 15.0 + 1.7 ab 15.9 + 4.1 b
T_BAD3 (m2 ha-1) 7.4 + 0.4 a 6.1 + 0.5 ab 4.9 + 1.1 b 3.8 + 1.9 ab 5.0 + 0.8 ab 8.0 + 1.9 ab
T_BAD4 (m2 ha-1) 1.5 + 0.2 a 1.2 + 0.1 b 0.4 + 0.2 abc 0.5 + 0.3 abc 3.0 + 0.6 ac 1.7 + 1.0 abc
P_BAD1 0.15 + 0.02 a 0.31 + 0.02 b 0.33 + 0.04 b 0.27 + 0.08 ab 0.31 + 0.04 b 0.22 + 0.06 ab
P_BAD2 0.40 + 0.02 0.47 + 0.02 0.52 + 0.03 0.54 + 0.10 0.46 + 0.05 0.44 + 0.08
P_BAD3 0.35 + 0.02 a 0.20 + 0.02 b 0.14 + 0.03 b 0.16 + 0.07 ab 0.15 + 0.02 b 0.27 + 0.06 ab
P_BAD4 0.09 + 0.01 a 0.02 + 0.01 b 0.04 + 0.01 b 0.02 + 0.02 ab 0.05 + 0.02 ab 0.07 + 0.04 ab
TPH (trees ha-1) 825 + 79 a 2268 + 295 b 2334 + 386 b 1694 + 447 ab 2027 + 303 b 1354 + 249 b
T_TPHD1 (trees ha-1) 554 + 77 a 1895 + 293 b 1642 + 320 b 1340 + 477 ab 1638 + 320 b 914 + 239 ab
142
T_TPHD2 (trees ha-1) 203 + 15 a 317 + 20 b 598 + 189 b 318 + 51 ab 338 + 39 ab 364 + 105 ab
T_TPHD3 (trees ha-1) 62 + 4 54 + 5 93 + 49 33 + 17 43 + 7 69 + 15
T_TPHD4 (trees ha-1) 6 + 0.8 a 2 + 0.6 b 1 + 0.7 ab 2 + 2 ab 8 + 2 ab 7 + 5 ab
P_TPHD1 0.38 + 0.03 a 0.64 + 0.03 b 0.65 + 0.05 b 0.50 + 0.13 ab 0.60 + 0.06 b 0.50 + 0.10 ab
P_TPHD2 0.39 + 0.02 0.29 + 0.03 0.29 + 0.04 0.43 + 0.11 0.34 + 0.05 0.33 + 0.08
P_TPHD3 0.20 + 0.02 a 0.07 + 0.01 b 0.06 + 0.03 b 0.07 + 0.05 ab 0.03 + 0.01 b 0.15 + 0.07 ab
P_TPHD4 0.03 + 0.01 a <0.01 b <0.01 ab <0.01 ab <0.01 ab 0.02 + 0.01 ab
MN_CRVL (m3 tree-1) 141.2 + 8.4 a 68.8 + 4.8 b 69.8 + 13.3 b 77.2 + 20.8 ab 64.3 + 7.3 b 83.0 + 11.6 b
CRVL (m3 ha-1 x 103) 84.2 + 7.5 114.0 + 11.9 142.6 + 44.5 135.4 + 72.7 152.6 + 43.8 94.8 + 22.8
T_CRVLD1 (m3 ha-1x 103) 3.0 + 0.6 a 8.6 + 1.6 b 7.7 + 1.5 b 4.4 + 1.9 ab 6.4 + 1.3 b 6.1 + 2.1 ab
T_CRVLD2 (m3 ha-1x 103) 24.8 + 3.0 a 51.5 + 6.1 b 5.5 + 8.1 b 28.0 + 10.6 ab 32.8 + 6.2 ab 32.5 + 7.9 ab
T_CRVLD3 (m3 ha-1x 103) 33.5 + 3.2 47.0 + 7.6 71.7 + 40.7 64.8 + 47.0 52.5 + 13.7 47.0 + 16.0
T_CRVLD4 (m3 ha-1x 103) 23.0 + 5.2 a 6.9 + 2.9 b 7.4 + 4.8 b 38.2 + 27.2 ab 60.9 + 31.7 ab 8.5 + 5.2 ab
P_CRVLD1 0.04 + 0.01 a 0.09 + 0.02 b 0.08 + 0.02 b 0.08 + 0.05 ab 0.13 + 0.04 b 0.05 + 0.02 ab
P_CRVLD2 0.31 + 0.02 a 0.50 + 0.03 b 0.56 + 0.05 b 0.49 + 0.13 ab 0.44 + 0.05 ab 0.40 + 0.09 ab
P_CRVLD3 0.46 + 0.02 a 0.36 + 0.03 ab 0.29 + 0.05 b 0.33 + 0.12 ab 0.27 + 0.04 b 0.45 + 0.08 ab
P_CRVLD4 0.18 + 0.02 a 0.05 + 0.02 b 0.06 + 0.03 ab 0.11 + 0.09 ab 0.14 + 0.04 ab 0.09 + 0.05 ab
UNVG_5 0.14 + 0.01 ad 0.27 + 0.02 bc 0.37 + 0.03 b 0.25 + 0.1abcd 0.29 + 0.03 bc 0.20 + 0.04 cd
UNVG5_50 0.33 + 0.02 a 0.31 + 0.02 a 0.33 + 0.04 a 0.78 + 0.07 b 0.69 + 0.04 b 0.25 + 0.05 a
UNVG_50 0.08 + 0.01 0.09 + 0.01 0.07 + 0.01 0.10 + 0.05 0.15 + 0.02 0.06 + 0.02
143
UNVG_200 <0.01 b <0.01 b <0.01 b <0.01 ab 0.04 + 0.01 a <0.01 b
a Refer to Table 4 for meaning of variable abbreviations.
pine/Douglas-fir contained less basal area (T_BAD1, P_BAD1, T_BAD2, and
P_BAD2), lower tree density (T_TPHD1, P_TPHD1, and T_TPHD2), and less crown
volume (T_CRVLD1, P_CRVLD1, and T_CRVLD2, P_CRVLD2; Table 8). In the
two largest dbh categories (33.0-48.2 cm), ponderosa pine/Douglas-fir contained
greater basal area (P_BAD3, T_BAD4, and P_BAD4), greater tree density
(P_TPHD3, T_BAD4, and P_BAD4), and greater crown volume (P_CRVLD3,
T_CRVLD4, and P_CRVLD4) than Douglas-fir forests (Table 8). Similar to the
pattern described above, overstory structure of ponderosa pine/Douglas-fir forests
also differed from Douglas-fir/blue spruce and quaking aspen/blue spruce, but
primarily in the smallest and second-largest dbh categories (Table 8). Quaking aspen
and Douglas-fir/limber pine showed few differences in overstory structure compared
to ponderosa pine/Douglas-fir, probably because these forest types were least
sampled and had highest SE (Table 8). Proportion of understory vegetation 1-4 cm
tall (UNVG_5) was least in ponderosa pine/Douglas-fir compared to Douglas-fir,
Douglas-fir/blue spruce, and quaking aspen/blue spruce, while quaking aspen and
quaking aspen/blue spruce contained the highest proportion of understory vegetation
5-49 cm tall (UNVG_50; Table 8).
Comparisons Between Vegetation And Owl Demography
For comparisons with vegetation variables, I used two demographic variables
that differed significantly among the 12 territories: breeding yr, which was
equivalent to total nesting attempts, and owlets. While these variables were highly
correlated (r = 0.95) and resulted in similar territory rankings (Table 5), I retained
both variables in analyses to assess the efficacy of breeding yr as a surrogate for
145
owlets in future analyses of habitat quality. In addition to these variables, I also
included bachelor yr and occupied yr in comparisons with vegetation variables.
Bachelor yr, which was negatively correlated with owlets (-0.41) and breeding yr (-
0.40) and resulted in nearly opposite territory rankings (Table 5), was included to
compare habitat conditions in breeding vs non-breeding territories. Occupied yr was
included to determine if habitat quality could be inferred from duration of occupancy.
Comparisons of territory and non-territory habitat were based solely on owlets.
Correlation Between Forest Type And Demography.—Across territories, breeding yr
was positively correlated with total area and proportion of area in ponderosa
pine/Douglas-fir, and negatively correlated with proportion of area in Douglas-fir
(Table 9). Owlets showed the same, but slightly weaker, correlations (Table 9). In
contrast, bachelor yr was negatively correlated with total area and proportion of area
in ponderosa pine/Douglas-fir, and positively correlated with proportion of area in
Douglas-fir (Table 9). Occupied yr was correlated only with proportion of area in
ponderosa pine/Douglas-fir (Table 9). Significant correlations between forest types
and demographic variables generally showed strong linearity (Fig. 21).
Correlation Between Forest Structure And Demography.—Across territories, owlets
were positively correlated with basal area (P_BAD3) and crown volume (T_CRVLD3
and P_CRVLD3) in the second-largest dbh category (33.0-48.2 cm), and showed a
trend (P = 0.07-0.09) of negative correlations with several forest structure variables in
the two smallest dbh categories (2.5-17.7 cm and 17.8-32.9 cm; Table 10).
Associated with these overstory variables, partial correlation analyses identified a
single structural dimension associated with owlets that was dominated by crown
146
Figure 21. Correlations between demographic variables and forest types across owl
territories (n = 12).
y = 0.729x + 0.2794
R2 = 0.8171
0
2
4
6
8
10
12
14
16
18
0.0 5.0 10.0 15.0 20.0
Area (ha) in ponderosa pine/Douglas-fir
Bre
edin
g y
r
y = -0.086x + 10.101
R2 = 0.477
0
2
4
6
8
10
12
0.0 20.0 40.0 60.0 80.0 100.0
Percent of territory area in ponderosa pine/Douglas-fir
Bac
hel
or
yr
y = 1.3951x - 0.0644
R2 = 0.6491
0
5
10
15
20
25
30
35
40
0.0 5.0 10.0 15.0 20.0
Area (ha) in ponderosa pine/Douglas-fir
Ow
lets
Table 9. Correlations among demographic variables and forest types within owl territories (df = 11 for all correlations). Significant
correlations (p < 0.05) are indicated with an asterisk.
Owlets Breeding yr Bachelor yr Occupied yr
Forest Types r P r P r P r P
Pipo/psme1 (area ha) 0.81 0.002* 0.90 < 0.001* -0.56 0.06 0.66 0.02*
Pipo/psme(prop. of total7) 0.70 0.01* 0.76 0.004* -0.69 0.01* 0.43 0.16
Psme2 (area ha) -0.19 0.55 -0.29 0.64 0.26 0.42 -0.02 0.35
Psme (prop. of total) -0.58 0.05* -0.62 0.03* 0.65 0.02* -0.30 0.35
Psme/pipu3 (area ha) -0.19 0.67 -0.16 0.62 0.24 0.46 -0.02 0.91
Psme/pipu (prop. of total) -0.25 0.43 -0.29 0.36 0.54 0.07 -0.04 0.98
Potr4(area ha) 0.04 0.91 -0.03 0.93 -0.17 0.61 -0.12 0.71
Potr (prop. of total) -0.28 0.38 -0.31 0.34 -0.05 0.89 -0.35 0.26
Potr/pipu5 (area ha) -0.14 0.67 -0.01 0.97 -0.16 0.63 -0.10 0.76
Potr/pipu (prop. of total) -0.35 0.26 -0.37 0.24 0.19 0.55 -0.29 0.37
Psme/pifl6 (area ha) -0.15 0.65 -0.18 0.57 -0.05 0.88 -0.22 0.48
Psme/pifl (prop. of total) -0.20 0.53 -0.23 0.47 <0.01 0.98 -0.25 0.43
1 ponderosa pine/Douglas-fir; 2 Douglas-fir; 3 Douglas-fir/blue spruce; 4 quaking aspen; 5 quaking aspen/blue spruce; 6 Douglas-fir/limber pine; 7 proportion of total area (ha) in territory
148
Table 10. Correlations among demographic and forest structure variables within owl territories (df = 11 for all analyses). Significant
correlations (p < 0.05) are indicated with an asterisk.
Owlets Breeding yr Bachelor yr Occupied yr
Variablea r P r P r P r P
MN_DBH 0.54 0.07 0.51 0.09 -0.52 0.08 0.25 0.43
BA -0.21 0.51 -0.18 0.58 0.78 0.003* 0.25 0.43
T_BAD1 -0.38 0.22 -0.35 0.26 0.70 0.01* 0.02 0.96
T_BAD2 -0.34 0.28 -0.28 0.38 0.77 0.003* 0.13 0.68
T_BAD3 0.56 0.06 0.51 0.09 -0.38 0.22 0.34 0.28
T_BAD4 0.19 0.55 0.15 0.65 -0.39 0.21 -0.06 0.86
P_BAD1 -0.51 0.09 -0.45 0.14 0.52 0.08 -0.19 0.55
P_BAD2 -0.46 0.13 -0.42 0.18 0.53 0.08 -0.15 0.64
P_BAD3 0.58 0.05* 0.52 0.09 -0.51 0.09 0.27 0.40
P_BAD4 0.18 0.58 0.17 0.60 -0.53 0.08 -0.12 0.71
TPH -0.26 0.42 -0.30 0.34 0.56 0.06 <0.01 0.99
T_TPHD1 -0.19 0.55 -0.23 0.47 0.48 0.11 0.03 0.93
T_TPHD2 -0.53 0.08 -0.57 0.05* 0.78 0.003* -0.17 0.59
T_TPHD3 -0.02 0.95 -0.12 0.71 0.02 0.96 -0.12 0.71
T_TPHD4 -0.06 0.85 -0.13 0.69 <0.01 0.99 -0.14 0.67
149
P_TPHD1 -0.55 0.07 -0.50 0.10 0.34 0.28 -0.35 0.27
P_TPHD2 -0.30 0.35 0.33 0.30 0.11 0.73 0.42 0.18
P_TPHD3 0.55 0.06 0.48 0.12 -0.42 0.18 0.28 0.38
P_TPHD4 0.22 0.49 0.22 0.49 -0.50 0.10 -0.05 0.86
MN_CRVL 0.23 0.47 0.18 0.58 -0.50 0.10 -0.09 0.78
CRVL 0.05 0.87 -0.09 0.78 0.22 0.49 0.03 0.93
T_CRVLD1 -0.44 0.15 -0.45 0.15 0.67 0.02* -0.10 0.75
T_CRVLD2 -0.51 0.09 -0.55 0.07 0.69 0.01* -0.20 0.52
T_CRVLD3 0.65 0.02* 0.50 0.10 -0.44 0.16 0.29 0.34
T_CRVLD4 -0.03 0.93 -0.08 0.80 0.05 0.87 -0.06 0.86
P_CRVLD1 -0.25 0.44 -0.09 0.79 0.26 0.42 0.05 0.88
P_CRVLD2 -0.51 0.09 -0.48 0.11 0.64 0.02* -0.16 0.61
P_CRVLD3 0.67 0.02* 0.65 0.02* -0.71 0.01* 0.30 0.34
P_CRVLD4 -0.04 0.90 -0.14 0.67 -0.10 0.75 -0.21 0.52
UNVG_5 -0.50 0.10 -0.50 0.10 0.25 0.42 -0.39 0.21
UNVG5_50 -0.09 0.78 -0.09 0.78 -0.11 0.73 -0.16 0.61
UNVG_50 0.07 0.83 0.11 0.73 -0.11 0.73 0.06 0.85
UNVG_200 -0.08 0.80 -0.02 0.94 -0.11 0.73 -0.08 0.79
a Refer to Table 4 for meaning of variable abbreviations.
volume (Step 1 variable = P_CRVLD3, partial r = 0.67, and Step 2 variable =
T_CRVLD3, partial r = 0.09). Compared with owlets, breeding yr showed similar
correlations but they were generally weaker (Table 10). In contrast, bachelor yr was
positively correlated with basal area (BA; Table 10). In the two smallest dbh
categories, bachelor yr was also positively correlated with basal area (T_BAD1, and
T_BAD2), tree density (T_TPHD2), and crown volume (T_CRVLD1, T_CRVLD2,
and P_CRVLD2), and was negatively correlated with crown volume (P_CRVLD3) in
the second-largest dbh category (Table 10). Associated with all significant overstory
variables, partial correlation analyses identified a single structural dimension
correlated with bachelor yr that was dominated by basal area (Step 1 variable = BA,
partial r = 0.78; Step 2 variable = T_TPHD2, partial r = 0.11). Occupied yr was not
correlated with any forest structure variables (Table 10).
Comparison Between Territory and Non-territory Habitat.—Territory habitat
(territories combined) differed from non-territory habitat in having less total basal
area (BA), and in containing less basal area (T_BAD2) and lower tree density
(T_TPHD2 and P_TPHD2) in the second-smallest dbh category (Table 11). Territory
habitat also contained a higher proportion of understory vegetation 5-49 cm tall
(UNVG_50) than non-territory habitat (Table 11).
Comparisons Among Territory Classes and Non-territory Habitat.—Compared with
non-territory habitat, the high-productivity class (A4 territory) was characterized as
having trees with larger overall mean dbh (MN_DBH), and in the second-smallest
dbh category, having less basal area (T_BAD2 and P_BAD2), lower tree density
(T_TPHD2), and less crown volume (T_CRVLD2 and P_CRVLD2; Table 12). The
151
Table 11. Comparison of forest structure variables (mean + SE) in territory vs non-
territory habitat (ANOVA tests, df = 351). Significant correlations (P < 0.05) are
indicated with an asterisk.
Variablea (units) Territory Non-territory F P
MN_DBH (cm) 28.5 + 0.6 27.3 + 0.6 1.76 0.18
BA (m2 ha-1) 25.2 + 0.8 28.4 + 1.2 4.97 0.03*
T_BAD1 (m2 ha-1) 6.7 + 0.6 6.1 + 0.6 2.92 0.09
T_BAD2 (m2 ha-1) 11.1 + 0.6 13.9 + 0.9 7.49 0.007*
T_BAD3 (m2 ha-1) 6.5 + 0.3 7.1 + 0.6 1.02 0.31
T_BAD4 (m2 ha-1) 1.1 + < 0.1 1.4 + < 0.1 1.70 0.19
P_BAD1 0.23 + 0.02 0.21 + 0.02 0.78 0.18
P_BAD2 0.41 + 0.02 0.46 + 0.02 3.10 0.08
P_BAD3 0.30 + 0.02 0.26 + 0.02 3.06 0.08
P_BAD4 0.06 + 0.01 0.06 + 0.01 0.02 0.89
TPH (trees ha-1) 1372.8 + 130.6 1426.2 + 153.6 0.26 0.61
T_TPHD1 (trees ha-1) 1072.5 + 127.2 1015.4 + 145.2 2.54 0.11
T_TPHD2 (trees ha-1) 241.1 + 14.8 333.2 + 43.7 5.19 0.02*
T_TPHD3 (trees ha-1) 54.8 + 3.0 71.4 + 11.1 2.89 0.09
T_TPHD4 (trees ha-1) 4.5 + 0.5 5.9 + 1.0 1.48 0.22
P_TPHD1 0.50 + 0.02 0.46 + 0.03 2.02 0.16
P_TPHD2 0.32 + 0.02 0.40 + 0.03 5.14 0.02*
P_TPHD3 0.16 + 0.02 0.12 + 0.01 2.52 0.14
152
P_TPHD4 0.02 + 0.006 0.02 + 0.004 0.82 0.37
MN_CRVL (m3 tree-1) 117.3 + 7.4 99 + 6.1 1.93 0.17
CRVL (m3 ha-1 x 103) 92.8 + 0.7 116.6 + 16.3 1.01 0.32
T_CRVLD1 (m3 ha-1 x 103) 5.0 + 0.6 5.1 + 1.0 0.50 0.48
T_CRVLD2 (m3 ha-1 x 103) 33.8 + 3.2 33.8 + 4.0 0.14 0.71
T_CRVLD3 (m3 ha-1 x 103) 38.9 + 3.8 48.6 + 10.0 0.03 0.86
T_CRVLD4 (m3 ha-1 x 103) 14.9 + 2.8 29.0 + 10.0 2.55 0.11
P_CRVLD1 0.07 + 0.01 0.05 + 0.01 0.75 0.39
P_CRVLD2 0.37 + 0.02 0.42 + 0.03 2.02 0.16
P_CRVLD3 0.43 + 0.02 0.39 + 0.03 1.92 0.17
P_CRVLD4 0.13 + 0.02 0.13 + 0.02 0.00 0.98
UNVG_5 0.20 + 0.01 0.20 + 0.01 0.06 0.81
UNVG5_50 0.40 + 0.02 0.30 + 0.02 13.08 0.0003*
UNVG_200 0.08 + 0.01 0.09 + 0.01 0.35 0.55
UNVG_300 <0.01 + <0.01 <0.01 + <0.01 0.47 0.49
a Refer to Table 4 for meaning of variable abbreviations.
Table 12. Comparison of forest structure variables (mean + SE) among non-territory
and three classes of territory habitat. Different letters denote significant differences
among habitats (Tukey’s tests, df = 348, p = 0.05).
Class of Habitat2
Variable1 (units) High Moderate
Low Non-territory
MN_DBH (cm) 35.0 + 2.5 a 29.0 + 0.7 b 26.0 + 0.8 c 27.0 + 0.6 bc
BA (m2 ha-1) 20.4 + 1.5 a 25.7 + 1.2 ab 26.2 + 1.2 ab 28.4 + 1.2 b
T_BAD1 (m2 ha-1) 3.2 + 0.9 ac 6.8 + 0.8 bc 8.3 + 0.9 b 6.1 + 0.7 c
T_BAD2 (m2 ha-1) 7.6 + 1.4 a 11.1 + 3.5 ab 12.2 + 0.8 ab 13.9 + 0.9 b
T_BAD3 (m2 ha-1) 7.6 + 1.0 ab 7.0 + 0.4 a 4.7 + 0.4 b 7.1 + 0.6 a
T_BAD4 (m2 ha-1) 2.1 + 0.5 1.0 + 0.2 1.0 + 0.2 1.4 + 0.2
P_BAD1 0.14 + 0.04 a 0.23 + 0.02 ab 0.28 + 0.02 b 0.21 + 0.02 a
P_BAD2 0.32 + 0.05 a 0.40 + 0.02 ab 0.45 + 0.02 ab 0.46 + 0.02 b
P_BAD3 0.42 + 0.05 a 0.32 + 0.02 ac 0.21 + 0.02 bc 0.26 + 0.02 c
P_BAD4 0.11 + 0.03 0.06 + 0.01 0.05 + 0.01 0.06 + 0.01
TPH (trees ha-1) 926 + 247 a 1341 + 196 ab 1704 + 191 b 1426 + 154 ab
T_TPHD1 (trees ha-1) 694 + 247 ad 1033 + 190 bc 1346 + 187 b 1015 + 145 cd
T_TPHD2 (trees ha-1) 163 + 32 a 244 + 21 ab 305 + 26 b 333 + 44 b
T_TPHD3 (trees ha-1) 60 + 8 59 + 4 48 + 5 71 + 11
T_TPHD4 (trees ha-1) 9 + 2 4 + 1 5 + 1 6 + 1
P_TPHD1 0.29 + 0.08 ad 0.50 + 0.04 bc 0.60 + 0.03 b 0.46 + 0.03 cd
P_TPHD2 0.34 + 0.06 0.32 + 0.03 0.30 + 0.03 0.40 + 0.03
P_TPHD3 0.32 + 0.06 a 0.16 + 0.02 b 0.09 + 0.02 b 0.12 + 0.01 b
P_TPHD4 0.06 + 0.02 a 0.02 + 0.01 b 0.02 + 0.01 b 0.02 + 0.003 b
MN_CRVL (m3 tree-1) 175.0 + 23.8 a 111.5 + 8.8 ab 106.6 + 12.4 b 99.0 + 6.1 b
CRVL (m3 ha-1 x 103) 105.5 + 15.2 89.5 + 5.5 101.2 + 9.3 116.7 + 16.3
T_CRVLD1 (m3 ha-1 x 103) 2.7 + 1.0 ad 5.2 + 0.8 bc 6.4 + 1.50 b 5.1 + 1.0 cd
T_CRVLD2 (m3 ha-1 x 103) 15.3 + 4.2 a 32.0 + 4.3 ac 43.3 + 5.2 b 33.9 + 3.7 bc
T_CRVLD3 (m3 ha-1 x 103) 63.7 + 23.8 39.3 + 5.0 35.4 + 4.8 48.6 + 9.9
T_CRVLD4 (m3 ha-1 x 103) 23.8 + 11.7 12.9 + 3.6 16.1 + 4.5 29.0 + 10.0
P_CRVLD1 0.03 + 0.02 0.07 + 0.01 0.08 + 0.01 0.05 + 0.01
P_CRVLD2 0.23 + 0.05 a 0.33 + 0.03 ac 0.47 + 0.03 b 0.42 + 0.03 bc
P_CRVLD3 0.53 + 0.05 a 0.47 + 0.03 a 0.32 + 0.03 b 0.39 + 0.03 ab
P_CRVLD4 0.20 + 0.04 0.12 + 0.02 0.13 + 0.03 0.13 + 0.02
UNVG_5 0.15 + 0.03 a 0.19 + 0.01 a 0.24 + 0.02 b 0.20 + 0.01 ab
UNVG5_50 0.42 + 0.05 ab? 0.40 + 0.03 a 0.40 + 0.03 a 0.30 + 0.02 b
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UNVG_50 0.09 + 0.02 0.08 + 0.01 0.06 + 0.01 0.09 + 0.01
UNVG_200 0.01 + .001 0.01 + 0.002 0.01 + 0.001 0.01 + 0.004
1 Refer to Table 4 for meaning of variable abbreviations.
2 High = high-productivity (territories producing > 23 owlets); moderate = moderate-
productivity (territories producing 13-23 owlets); low = low-productivity (territories
producing < 9 owlets);
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high-productivity class also had a greater proportion of basal area (P_BAD3) in the
second-largest dbh category, and greater tree density (P_TPHD3 and P_TPHD4) in
the two largest dbh categories, than non-territory habitat (Table 12). Notably, the
moderate-productivity class was not different than non-territory habitat for any
forest structure variable (Table 12). Compared with non-territory habitat, the low-
productivity class differed in the smallest dbh category by containing greater basal
area (T_BAD1 and P_BAD1), higher tree density (T_TPHD1 and P_TPHD1), and
greater crown volume (T_CRVLD1), and differed in the second-largest dbh category
by containing less basal area (T_BAD3; Table 12). The low-productivity class also
contained a greater proportion of understory vegetation 1-4 cm tall (UNVG_5) than
non-territory habitat (Table 12).
Possible Limiting Factors Associated With Habitat Relationships
Density of Cavity Trees.—Territories contained a mean 7.7 + 1.0 cavity trees (range 3
– 13; Table 13), but density of cavity trees was not correlated with owlets (R2 = 0.01,
F = 0.05, df = 11, P = 0.83). Mean density of cavity trees across territories was 0.6 +
0.3 ha-1 (range 0.3 – 1.0 ha–1; Table 13). However, non-territory habitat contained
only 0.05 cavity trees ha-1, less than 10% of mean cavity-tree density within
territories (Table 13, Fig. 20). Moreover, nearly 30% (5 of 17) of all non-territory
cavities occurred in a single cluster within the boundaries of an unstudied territory
(A40) on the edge of the study area (Fig. 20). Cavity trees generally had a clustered
distribution across the study area because most cavity trees were quaking aspen (74
%; 89 of 120) that occurred in discrete patches along drainage bottoms. As a result,
several portions of the study area contained no cavity trees (Fig. 20).
156
Table 13. Density of cavity trees among owl territories and in non-territory habitat.
Territory Total cavity trees Territory area (ha) Density of cavity trees
(trees ha-1)
A2 3 6.6 0.46
A4 9 21.3 0.42
A7 4 11.1 0.36
A8 8 17.6 0.45
A10 13 18.5 0.70
A11 13 12.5 1.04
A12 6 6.6 0.91
A13 11 20.6 0.53
A18 7 17.8 0.39
A20 6 18.3 0.33
A27 5 5.4 0.92
A29 7 24.8 0.28
MEAN + SE 7.7 + 1.0 15.1 + 1.9 0.6 + 0.1
NON-TERRITORY 17 318.51 0.05
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Predation on Artificial Nests.—Predation rates on artificial nests differed among
territory classes (χ2 = 15.5, df = 2, P < 0.001), but there was no clear pattern.
Predation rates were highest in moderate-productivity territories (42%; 35 of 84 nest-
weeks), lowest in low-productivity territories (12%; 10 of 84), and intermediate in
high-productivity territories (21%; 15 of 74). As a result, predation rates were not
correlated with owlets (R2 = 0.03, F = 0.11, df = 5, P = 0.76). Predation rates were
24% (20 of 84 nest-weeks) in ponderosa pine/Douglas-fir, 30% (22 of 73)
in Douglas-fir, and 23% (18 of 77) in quaking aspen/blue spruce. Predation rates did
not differ among forest types (χ2 = 1.08, df = 2, P = 0.58).
Density of Red Squirrel middens did not differ among territory classes (F =
0.39, df = 5, P = 0.71). Mean density of middens was 0.8 + 0.1 ha-1 in high-
productivity territories, 1.1 + 0.1 ha-1 in moderate-productivity territories, and 1.1 +
0.1 ha-1 in low-productivity territories. However, frequency of middens differed
among forest types (χ2 = 14.75, df = 4, P =0.005). Frequency of middens was highest
in Douglas-fir (35 middens, 28 expected based on proportion of area in Douglas-fir),
Douglas-fir/blue spruce (17 middens, 7.2 expected), quaking aspen (11 middens, 3.3
expected), and quaking aspen/blue spruce (4 middens, 0.1 expected), and least in
ponderosa pine/Douglas-fir (50 middens, 67.6 expected) and Douglas-fir/limber pine
(no middens, 0.1 expected). Midden density was not correlated with owlets (R2 =
0.14, F = 0.63, df = 5, P = 0.47) or with predation rates at artificial nests (R2 = 0.03, F
= 0.13, df = 5, P = 0.74).
Relative Prey Abundance.—Arthropod abundance did not differ between the high-
productivity territory (A4) and the low-productivity territory (A18; Table 14).
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Table 14. Comparison of relative arthropod abundance (mean + SE) between a high-
productivity territory (A4) and a low-productivity territory (A18) during 1998-1999
(statistical significance based on ANOVA).
No. arthropods (mean + SE)
trap-night-1
Nesting stage Forest type1 A4 territory A18 territory df F p
Incubation Pipo/psme 14.0 + 3.4 12.8 + 4.3 7 0.05 0.83
Psme 13.8 + 4.7 11.8 + 4.8 7 0.09 0.78
Potr/pipu 8.9 + 2.2 11.4 + 4.5 7 0.24 0.65
Nestling Pipo/psme 11.0 + 1.8 13.4 + 2.1 9 0.77 0.40
Psme 21.4 + 5.7 20.4 + 2.9 9 0.03 0.88
Potr/pipu 24.1 + 7.6 21.7 + 2.2 8 0.07 0.80
1 Pipo/psme = ponderosa pine/Douglas-fir; Psme = Douglas-fir; Potr/pipu = quaking
aspen/blue spruce
159
DISCUSSION
Habitat Quality
Territory quality of Flammulated Owls in this study was associated with
mature forests of ponderosa pine/Douglas-fir. Owlets and breeding yr (equivalent to
total nesting attempts), which differed markedly among territories, were positively
correlated with territory area and proportion of area in ponderosa pine/Douglas-fir
forests. Territory quality was also associated with overstory structure in these forests,
based on the fact that owlets were positively correlated with greater crown volume in
the second-largest (33.0-48.2 cm) dbh category. In contrast, Douglas-fir forests were
sub-optimal habitat for breeding owls. Owlets and breeding yr were negatively
correlated with proportion of area in these forests, which contained greater tree
density, basal area, and crown volume in the two smallest dbh categories (2.5-17.7
and 17.8-32.9 cm) than ponderosa pine/Douglas-fir. Moreover, bachelor yr (total yr
territories were occupied by unpaired males) was positively correlated with this forest
type and with basal area. Previous studies found that unpaired males of other species
including raptors occupied territories in sub-optimal habitats (Newton and Marquiss
1976, Korpimaki 1988, Burke and Nol 1998).
Density of cavity trees was not correlated with reproductive success,
indicating that abundance of cavity trees was not associated with territory quality. All
territories contained > 3 cavity trees and had > 1 breeding attempt over the study,
suggesting that relatively few cavity trees were sufficient for nesting. However,
density of cavity trees clearly distinguished territory from non-territory habitat. Non-
territory habitat not only had a cavity-tree density that was < 10% of mean density
160
within territories, but because cavity-trees generally had a clustered distribution
across the study area, most of the non-territory habitat was characterized as
containing no cavity-trees.
The apparent importance of cavity trees in territory occupancy by males was
underscored by the few differences in other aspects of forest structure that
distinguished territory from non-territory habitat overall. Compared to non-territory
habitat, combined territories contained fewer trees and less basal area in the second-
smallest dbh category, and more understory cover 5-49 cm tall. However, in
comparisons of the three classes of territory productivity (high-, moderate-, and low-
productivity) with non-territory habitat, only the high-productivity class (which
contained only A4 territory) was distinguished by having less basal area and lower
tree density in the smaller dbh categories and the converse in the larger dbh
categories. These data indicate that differences between combined territories and
non-territory habitat were attributable to the unique characterisitics of A4 territory,
and to differences between the two statistical analyses (compared with the two-class
analyses, the four-class analyses had a larger mean squared error and a higher critical
value for evaluation). Based on the fact that moderate-productivity territories showed
no differences in overstory structure from non-territory habitat, and that low-
productivity territories actually contained greater basal area and tree density in the
smaller dbh categories than non-territory habitat, availability of cavity trees appeared
fundamentally important for territory establishment by males. These data also
suggest that at least some portions of non-territory habitat, which overall contained
similar proportions of each forest type to combined territories, were potentially
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suitable for occupancy by males except for scarcity of cavity trees. Thus, despite the
fact that snags and cavity trees were relatively abundant on the study area, which had
not been harvested since the 1800s (Reynolds et al. 1985), cavity-tree availability
clearly affected owl distribution and density, as it has with many other secondary-
cavity nesters (e.g., Brawn and Balda 1988, Newton 1998 and sources cited therein).
In summary, my results indicate that habitat quality was determined by two
primary factors. First, cavity-tree availability determined where owls established
territories, and second, forest type and structure determined whether a territory was
more often occupied by breeding pairs or by bachelor males. High-quality breeding
habitat for Flammulated Owls in this study was characterized as mature, relatively
open stands of ponderosa pine/Douglas-fir that contained sufficient cavity trees for
nesting.
Uniqueness of A4 Territory
A4 territory was by far the most productive (35 owlets, 16 breeding yr) of all
territories, and contained significantly greater basal area, crown volume, and tree
density in the second-largest dbh category than the moderate and low-productivity
classes. Was the habitat in A4 territory optimal for Flammulated Owls? Given its
uniqueness I cannot be certain, but several observations indicate this may be true.
First, correlations between reproductive success on A4 territory and forest type and
forest structure represented a positive linear extension of patterns across all other
territories. Second, duration of this study was likely sufficient to dilute chance effects
associated with quality of individual owls. In fact, A4 territory was occupied by 8
unique females and 5 unique males over the 19 yr study, both of which were more
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than any other territory. Finally, A4 territory was the recipient of the most dispersals
(3; 2 females and 1 male), suggesting that this territory was preferable for breeding.
Possible Factors Underlying Habitat Relationships
Nest predation is a primary cause of nesting mortality for many bird species
(e.g., Skutch 1949, Ricklefs 1969), and is believed to be an important factor in the
evolution of life-history characteristics and habitat selection (Slagsvold 1982,
Sonerud 1985a, Martin 1988, Bosque and Bosque 1995). However, my data suggest
that nest predation was not associated with territory quality in Flammulated Owls. In
addition to the fact that nest success across territories was high (82% over the study),
as it is in most cavity-nesters compared with open nesting birds (Ricklefs 1969,
Wilcove 1985), I found no differences in predation rates among owl territories, and
no differences among forest types where nests were located. Density of Red Squirrel
middens, which I used as a measure of relative squirrel density in territories, was
highest in Douglas-fir forests where trees had highest density. This is consistent with
other studies that found denser stands of coniferous forests were preferred habitat for
Red Squirrels (Rusch and Reeder 1978, Gurnell 1984). However, density of middens
was not correlated with owl productivity, providing no evidence that Red Squirrels
affected habitat selection by owls. These results contrast with those of Korpimaki
(1993) and Sonerud (1985b), who found that rates of nest predation were high in
populations of cavity-nesting Tengmalm’s Owl (Aegolius funereus), and that owls
preferentially nested in areas where nest boxes had lowest predation rates.
Prey abundance affects the quality of breeding habitats for many birds
including raptors (e.g., Janes 1984, Korpimaki 1988, Burke and Nol 1998). However,
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I found no evidence that arthropod abundance over the 2 yr sampling period differed
between a high-productivity and low-productivity territory. Moreover, annual
constancy in owl reproductive variables (Chapter 2) suggests that any stochasticity
associated with arthropod abundance during the breeding season has had little effect
on long-term productivity. Still, research on prey abundance is needed over more yr
to better assess the relationship between prey and productivity, in addition to two
other aspects of prey abundance. Because topographic temperature gradients often
become established after dark (pers. observ.), study is needed to determine if flying
arthropods follow warmer temperatures by migrating to higher slope positions, where
ponderosa pine/Douglas-fir forests typically occur. In addition, because energetic
demands by females peak during egg-formation (generally late May in Colorado),
when nights are cold and arthropod activity is low, more study is needed to determine
if arthropod abundance during this possible ‘bottleneck’ period is correlated with owl
productivity.
Positive correlations between productivity and ponderosa pine/Douglas-fir
forests likely reflect the importance of these forests and their structure in the
behavioral ecology of Flammulated Owls. I previously reported that male owls,
which provide almost all the food for their mates and owlets until fledging, foraged
significantly more often in forests of ponderosa pine/Douglas-fir than in other forest
types (Linkhart et al. 1998). This forest type may be important for males because the
characteristically large, open tree crowns facilitate their gleaning and hover-gleaning
foraging tactics within and on the surface of tree crowns (Linkhart et al. 1998,
Reynolds and Linkhart 1987). Thus, as long as arthropod density is not limiting,
164
relative arthropod abundance among forest types or among territories may not be as
important as vegetation structure that facilitates capture of arthropods. Structure
associated with forests of ponderosa pine/Douglas-fir is also likely important to other
behaviors; I reported previously that owls preferentially used older (larger) ponderosa
pine and Douglas-fir trees for singing and day-roosting, probably because these trees
provided more protective cover against inclement weather and predators than younger
trees (Linkhart et al. 1998).
Bases for Inferring Territory Quality
Many studies have inferred habitat quality based on relative abundance (see
sources cited in Introduction) or duration of occupancy (e.g., Moller 1983, Bunzel
and Druke 1989, Newton 1989, Matthysen 1990). In my study occupied yr, which
was equivalent to duration of occupancy by breeding pairs and bachelor males, was
correlated only with territory area in ponderosa pine/Douglas-fir forest and showed
no correlation with any forest structure variables. This was because bachelor males
that occupied up to 70% of territories annually (Chapter 2) were positively associated
with sub-optimal habitat conditions. Therefore, inferring habitat quality based on
density or occupancy alone may be unreliable if mating status or reproductive success
of individual males is not known, as has been previously suggested (e.g., Van Horne
1983, Robinson 1992).
Breeding yr, which was identical to total nesting attempts in this study, was
nearly equivalent to total owlets fledged in habitat correlations because (1) nesting
success was high (82%), since relatively few nests were lost to predators, (2) range
of clutch size was very small (clutches almost always contained either 2 or 3 eggs),
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among the least of North American strigiforms (Chapter 2), and (3) most eggs
hatched and survived to fledging (Chapter 2). Consequently, breeding yr may be a
good surrogate for owlets in inferences of habitat quality in other portion of the
Flammulated Owl’s range, if breeding yr is associated with productivity as it was in
my study. Pairing status was used in assessments of habitat quality in other studies,
although the equivalency of this variable to productivity was not known (e.g., Burke
and Nol 1998).
Mean tenure on individual territories, which I used as an estimate of survival,
did not differ among territories for either sex suggesting that territory quality did not
influence individual survival. These data should be viewed cautiously for males,
however, since they were based primarily on males that nested more than once and
not on bachelor males (capturing bachelor males was difficult). Nonetheless, tenure
of males that were unpaired up to 5 yr between nesting attempts was similar to males
who nested annually (Chapter 4), suggesting that survival did not differ between
bachelor and breeding males. For males, this may indicate that survival is more
influenced outside of the breeding season, such as during migration, when many
Neotropical migrants species suffer mortality (Gill 1999). For females, survival may
be affected more by the energetic cost associated with both migration and egg-laying
(Chapter 2) than by selection of breeding territory. Tenure or turnover on territories
was influenced by territory quality in other studies of habitat quality (Marquiss and
Newton 1982, Korpimaki 1988). While the number of unique pair bonds on
territories ranged from 1 (A24, A18, and A2 territories) to 10 (A4 territory), the
frequency of this variable was primarily a function of the total number of breeding
166
pairs (= breeding yr) and was not a reflection of mate fidelity. Mean bond duration
did not differ among territories, likely because bond duration was limited by tenure of
females on territories (Chapter 4) rather than territory quality. Other studies have
found that mate fidelity differed among territories, suggesting that mate fidelity was
associated with habitat quality (Newton and Marquiss 1982, Haig and Oring 1988,
Part 1994).
The fact that most territories eventually produced owlets indicates that long-
term demographic data are needed to make accurate inferences regarding habitat
quality. While low-productivity territories were typically occupied by bachelors
annually, occasionally these territories were occupied by males that bred for 1-3
consecutive years. Short-term studies that captured only the years in which breeding
occurred on these territories would have identified relative territory quality much
differently than I have over the 19 yr study, resulting in inaccurate interpretations of
habitat quality. In addition, because most Flammulated Owls remained on territories
their entire known reproductive lives (up to 12 yr; Chapter 4), and because lifetime
reproduction was primarily a function of longevity (Chapter 3), long-term study was
required to assess contributions of individuals. Other studies have shown that long
time periods are necessary to evaluate effects of atypical individuals and extreme
environmental conditions (e.g., Woolfenden and Fitzpatrick 1984, Van Horne et al.
1997).
Stability of Territories
Considering the marked variation in reproductive success among territories,
frequent changes in territory boundaries and usurpation of territory ownership might
167
be expected if males attempted to improve their fitness (Janes 1984). However,
territory boundaries did not often change, possibly due to stable densities of cavity
trees, since other studies have found that territory boundaries shifted in response to
changing cavity resources (van Balen et al. 1982, East and Perrins 1988). But why
would males occupy low-quality territories their entire known reproductive lives
without dispersing to higher-quality territories? One possibility is that it is more
profitable for a male of a long-lived species such as the Flammulated Owl to await
the probable arrival of a female in a territory where he is familiar with the location of
sites for feeding, nesting, and escaping predators (Greenwood 1980). In addition,
males may "hedge their bets" by engaging in extra-pair copulations (EPCs; Reynolds
and Linkhart 1990, pers. observ.). Other raptor studies have found that territory
boundaries remained fixed or changed little year-to-year despite turnover of territory
occupants (Southern 1970, Newton 1976, Janes 1984, Nicholls and Fuller 1987), or
that males remained on the same territories even when higher-quality territories were
apparently available (e.g., Janes 1984, Woolfenden and Fitzpatrick 1984, Korpimaki
1988).
Distribution of Territories on the Landscape
Several authors reported finding clusters of singing male Flammulated Owls
separated by relatively large areas of apparently unoccupied habitat (e.g., Marcot and
Hill 1980, Howie and Ritcey 1987), a phenomenon that led some previous researchers
to postulate that the owls may be semi-colonial (see Winter 1974). Such clusters do
not provide direct evidence for coloniality, because observations are not based on
locations of nests but rather on responsiveness of singing males, which may or may
168
not be nesting. Compared to nesting males, unpaired males are more likely to be
detected because they sing with greater duration through the night and through the
breeding season (Linkhart et al. 1998). Aggregations of nesting territories, where
they exist, may reflect either surrounding areas of suitable habitat that are unoccupied
(Winter 1974), or surrounding habitat that appears suitable for breeding, but is in fact
suboptimal (Howie and Ritcey 1987, Reynolds and Linkhart 1992). My data support
the latter hypothesis, because male owls in this study only established territories
where suitable cavity trees for nesting were available, leaving unoccupied some
surrounding areas of habitat that appeared potentially suitable for breeding (i.e.,
forests consisting of mature, relatively open ponderosa pine/Douglas-fir), but lacked
cavity trees. Although density of cavity trees was not correlated with reproductive
success, territories were generally aggregated around clusters of cavities, which
occurred primarily in large quaking aspen trees in bottom areas, and secondarily in
large conifers on ridge tops (Reynolds et al. 1985, pers. observ.).
Forest Management
The correlation of productivity on territories with higher densities of larger-
diameter trees suggests that Flammulated Owls are adapted to forests that were
historically maintained by fire. Fire supression in many western forests, which were
characterized by open stands of large-diameter trees prior to European settlement, has
resulted in higher tree densities especially in the smaller diameter classes and has
resulted in conversion of many pine forests to fir forests (Cooper 1960, Covington
and Moore 1992). Tree density in smaller diameter classes was negatively correlated
with productivity on territories, suggesting that fire suppression may be resulting in
169
sub-optimal habitat for Flammulated Owls. Research is needed to determine the
effects on owl productivity of prescribed burns and/or selective logging that return
forest structure to pre-settlement conditions.
Further research is required to determine how and if patterns evident on this
511 ha study area are applicable elsewhere. Generally, breeding Flammulated Owls
have been associated with mature montane forests throughout their range (McCallum
1994a). However, Flammulated Owls may not be restricted to breeding in forests of
ponderosa pine/Douglas-fir, as indicated by studies that found owls breed in nest
boxes at relatively high densities in quaking aspen stands of northern Utah (Marti
1997), and that owls breed in pure forests of Douglas-fir in Montana (Powers et al.
1996). In order to determine the importance of floristics and forest structure, and to
assess the effects of forest management activities on breeding Flammulated Owls
over their range, researchers need to undertake comparative demographic studies of
owls in different forest types and across multiple forest management regimes.
170
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CHAPTER VI
CONCLUSIONS
Several demographic characteristics indicated that Flammulated Owls have a
life history more typical of larger birds, which generally have lower fecundity and
longer breeding lifespans, higher nesting success, and are longer-lived than smaller
birds (Newton 1998, Ricklefs 2000). First, fecundity of Flammulated Owls is among
the lowest and least variable of North American and European strigiforms, despite the
fact that they are one of the smallest (Johnsgard 1988). Second, male Flammulated
Owls had longer breeding lifespans compared to other species of strigiforms for
which comparable data exist (Korpimaki 1992, Marti 1997). Third, at least 75% of
nests were successful in 16 of 19 years for an overall nesting success rate of 82% over
the study. Among North American strigiforms, only Spotted Owls (S. occidentalis)
have a higher reported nesting success (85%; Forsman et al. 1984). Fourth, I never
observed replacement clutches or multiple broods in Flammulated Owls. Among
raptors in temperate regions, several small species and some larger species are known
to lay a replacement clutch if the first clutch is lost at an early stage (Newton 1979,
Johnsgard 1988). Finally, male Flammulated Owls have maximum longevity (at least
12 yr) greater than other small (< 150 g) owls and longevity comparable to many
larger owls (Glutz and Bauer 1980, Clapp et al. 1983, Klimkiewicz and Futcher 1989,
Klimkiewicz, pers. comm).
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Data on lifetime reproductive success (LRS) indicated that a small percentage
of adult Flammulated Owls accounted for the majority of the total offspring produced
in the population, which is consistent with LRS data from other avian species
(Clutton-Brock 1988, Newton 1989, Wiklund 1995). Seventeen percent of females
and 27% of males accounted for 50% of all Flammulated Owl offspring, while in
other species the percentages for females range from 15% (Red-billed Gulls; Mills
1989) to 31% (Kingfishers; Bunzel and Druke 1989), and the percentages for males
range from 14% (Indigo Buntings; Payne 1989) to 30% in (Kingfishers; Bunzel and
Druke 1989). Males had longer breeding lifespans and more mates over their
lifetimes than females. These differences may result from males having greater
longevity than females (see below). Total breeding years were strongly correlated
with lifetime productivity for females and males, because clutch sizes varied little and
nesting success was high. Among all bird species, breeding lifespan has emerged as
the major demographic determinant of LRS (Newton 1989).
Male Flammulated Owls had significantly longer tenure on territories than
females, probably because males had greater territory fidelity (98% vs 78%) and an
apparently longer lifespan. Male-biased site fidelity is widespread among raptors
(e.g., Newton and Marquiss 1982, Forero et al. 1999), because in resource-defense
mating systems (Emlen and Oring 1977) males may have more to gain by being
faithful to breeding territories than females (Greenwood 1980). While longer lifespan
in male Flammulated Owls has not been substantiated, it is plausible for two reasons.
First, mean longevity over 19 yr was significantly greater for males than females,
although this disparity may in part reflect female-biased dispersal. Second, unpaired
182
males annually occupied 10-70% of territories, which suggests a shortage of females
in the breeding population. The apparent lack of females may be tied to energetics.
Flammulated Owls lay clutches representing a greater proportion of their mass (55-
60%) than other North American strigiforms (Johnsgard 1988). Coupled with the
high energetic cost of long-distance migration immediately prior to egg-laying, these
data suggest that females may be predisposed to higher mortality rates than males.
Female Flammulated Owls had a higher rate of breeding dispersal than males
(0.22 vs 0.02), a pattern that is well-documented in other birds (e.g., Greenwood
1980, Marquiss and Newton 1982, Forero et al. 1999). Dispersing females moved to
territories where productivity over the 19 yr study was significantly greater than on
territories from which they dispersed, suggesting that females were capable of
distinguishing relative quality among territories.
Fidelity and dispersal data suggest that dispersal distance by female owls may
be bimodal with females moving to adjacent territories following successful nests and
more distant territories (> 2 km) following unsuccessful nests. Dispersal by females
to nearby territories, as was found in many other species (e.g., Payne and Payne 1996,
Hannon and Martin 1996, Williams 1996, Forero et al. 1999), may be beneficial
because dispersers can best judge the quality of resources and potential mates in
adjacent territories (Hinde 1956, Greenwood 1980, Ens et al. 1996). Although
dispersal following nesting failure is beneficial if chances of future nesting success
are improved (Murphy 1996), benefits of dispersing to more distant territories, where
owls are unlikely to have knowledge of resources or potential mates, are not clear.
183
Most male Flammulated Owls occupied a single territory their entire known
reproductive lives. Whether or not males were paired annually did not appear to limit
their tenure or likelihood of changing territories. Males often continued to occupy
original territories despite being unpaired up to four consecutive years, even when
more productive territories apparently were available. Thus, high site fidelity by
males appeared to counter predictions based on habitat selection models that assume
animals select habitats conferring highest reproductive success, and that if higher-
quality habitats become available individuals should move to the new sites (Fretwell
and Lucas 1969). Consequently, territory fidelity by Flammulated Owls may be
considered a suboptimal form of habitat selection with respect to territory quality
(Switzer 1993). Males of other species including raptors have been documented
remaining on a single territory, even when higher-quality territories apparently were
available (e.g., Janes 1984, Woolfenden and Fitzpatrick 1984, Korpimaki 1988).
Productivity of owls over the 19 yr study was positively correlated with
territory area in ponderosa pine/Douglas-fir forests, and with greater crown volume in
the second-largest of four tree-diameter categories. Productivity was not correlated
with density of cavity-trees. However, cavity trees clearly distinguished territory
from non-territory habitat, since non-territory habitat contained < 10% of mean
cavity-tree density within territories. Few structural characteristics distinguished
combined territories from non-territory habitat. In comparisons of non-territory
habitat with three classes of territory distinguished by differing productivity, only the
high-productivity class contained greater tree density and basal area in the larger dbh
categories. Based on the fact that moderate-productivity territories showed no
184
differences in forest structure from unoccupied habitat, and that low-productivity
territories (usually occupied by bachelor males) actually contained denser forests and
smaller trees than non-territory habitat, at least some portions of non-territory habitat
may have been suitable for establishment of territories except for scarcity of cavity
trees. Monitoring of predation on artificial nests for 1 yr and relative prey abundance
for 2 yr revealed no patterns among selected territories differing in productivity,
suggesting these factors were not associated with habitat quality.
My results indicate that habitat quality was determined by two primary
factors. First, cavity-tree availability determined where owls established territories,
and second, forest type and structure determined whether a territory was more often
occupied by breeding pairs or by bachelor males. High-quality breeding habitat for
Flammulated Owls in this study was characterized as mature, relatively open stands
of ponderosa pine/Douglas-fir that contained sufficient cavity trees for nesting.
Many studies have inferred habitat quality based on relative abundance or
duration of occupancy (e.g., Bunzel and Druke 1989, Newton 1989). In my study
occupied yr, which was equivalent to duration of occupancy by breeding pairs and
bachelor males, was correlated only with territory area in ponderosa pine/Douglas-fir
forest and showed no correlation with any forest structure variables. This was
because bachelor males that occupied up to 70% of territories annually were
positively associated with sub-optimal habitat conditions. Therefore, inferring habitat
quality based on density or occupancy alone may be unreliable or misleading if
mating status or reproductive success of individual males is not known, as has been
previously suggested (e.g., Van Horne 1983, Robinson 1992). Breeding yr may be a
185
good surrogate for productivity in future efforts to identify important breeding
habitats for this species, at least where other demographic parameters such as nesting
success are similar to mine.
Habitat correlations in this study suggest that environmental changes,
including fire suppression, may affect long-term viability of owl populations. The
open structure and species composition of ponderosa pine/Douglas-fir forests
throughout the western United States were historically maintained by frequent, low-
intensity ground fires (Cooper 1960). Fire suppression has resulted in increased tree
densities in ponderosa pine and mixed-conifer forests, has converted many pine
forests to fir forests, and has changed fire type from low-intensity to catastrophic,
habitat-destroying, crown fire (Barrett et al. 1980, Gordon 1980). In this study,
younger, dense stands of Douglas-fir were associated with sub-optimal breeding
habitat. In addition, many ponderosa pine and mixed-conifer forests within the range
of the Flammulated Owl have been harvested. These habitat changes have resulted in
declines of Flammulated Owls in some areas (e.g., Marshall 1957, 1988; Franzreb
and Ohmart 1978). Species exhibiting characteristics of K-selection (sensu Pianka
1970), such as Flammulated Owls, generally can be expected to respond slowly to
environmental perturbations because of their low fecundity and low density (Newton
1998). In order to understand effects of changes on structure and species composition
of owl breeding habitat, and on long-term impact to the reproduction and survival of
owl populations, researchers need to undertake comparative demographic studies of
owls across multiple forest management regimes and forest types.
186
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