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Habitat Selection by Spotted Owls during Natal Dispersal in Western OregonAuthor(s): Gary S. Miller, Robert J. Small and E. Charles MeslowSource: The Journal of Wildlife Management, Vol. 61, No. 1 (Jan., 1997), pp. 140-150Published by: Wiley on behalf of the Wildlife SocietyStable URL: http://www.jstor.org/stable/3802423 .

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HABITAT SELECTION BY SPOTTED OWLS DURING NATAL DISPERSAL IN WESTERN OREGON

GARY S. MILLER,' Cooperative Wildlife Research Unit, 104 Nash Hall, Oregon State University, Corvallis, OR 97331, USA ROBERT J. SMALL,2 Department of Wildlife Ecology, University of Wisconsin, Madison, WI 53706, USA E. CHARLES MESLOW,3 U.S. Fish and Wildlife Service, Cooperative Wildlife Research Unit, 104 Nash Hall, Oregon State

University, Corvallis, OR 97331, USA

Abstract: The probability of successful natal dispersal may influence the viability of northern spotted owl (Strix occidentalis caurina) populations within fragmented forests. We tested 6 null hypotheses examining the relations between habitat selection, forest fragmentation, and the probability of mortality for juvenile northern spotted owls during natal dispersal in western Oregon, 1982-85. Older forest (old-growth and mature stands) was the vegetation type used most frequently during transience (35.3%) and colonization (61.2%), and spotted owls selected closed-canopy forests over open-canopy forests during both phases of dispersal (P < 0.05). The hypotheses that spotted owl habitat selection was independent of forest fragmentation were not rejected for either transient (P = 0.51) or colonization (P = 0.43) dispersal. Likewise, net dispersal distance was independent of forest fragmentation (P = 0.92). Use of open sapling stands during transient dispersal decreased the probability of mortality (Wald test = 2.21, P = 0.03), whereas use of clearcuts during colonization dispersal increased the probability of mortality (Wald test = 1.95, P = 0.03). Net dispersal distance did not affect the probability of mortality (P = 0.67), yet a negative relation existed between dispersal distance and the amount of clearcut used during transient dispersal (P < 0.005). These results provide additional evidence of the selection of older forests by spotted owls and how their natal dispersal may be affected by harvesting such forests; particularly, that use of clearcuts may decrease the probability of successful natal dispersal.

J. WILDL. MANAGE. 61(1):140-150

Key words: dispersal, forest fragmentation, habitat selection, mortality, natal dispersal, Oregon, spotted owls, Strix occidentalis caurina, survival.

Estimates of habitat-specific demographic parameters are required to develop effective species-specific management and conservation

strategies (Van Horne 1983, Murphy and Noon 1991, Pulliam et al. 1992, Soule and Kohm 1989). The understanding of dispersal is severely limited when compared to survival and reproduction (Taylor 1990), and may preclude realistic efforts to manage for faunal movement across

landscapes (Harrison 1992). The importance of

dispersal in sustaining viable populations also is receiving increased attention in wildlife conservation (Kareiva 1990, Newton 1991a, Pul- liam and Danielson 1991), especially within

fragmented landscapes (Temple 1989, Weins 1990, Rolstad 1991).

The northern spotted owl is closely associated

with older-aged forests, and younger forests that contain residual older forest components (i.e., large trees, snags and downed woody material), in the Pacific Northwest and the species' decline is believed to be due to the loss and fragmentation of such forests (Thomas et al. 1990). De-

termining dispersal attributes and factors that influence survival during dispersal will become

increasingly important with increased fragmentation (Noon and Biles 1990). If owls select older forests during dispersal, the loss and fragmentation of those forests could reduce the probability of juveniles locating areas to establish a

breeding territory. Successful dispersal would decrease because juvenile owls would be re-

quired to disperse greater distances over poten- tially unsuitable habitat, during a period of high mortality (Gutibrrez et al. 1985, Miller and Mes- low 1985). Thus, as older forests become more scarce and more patchy in distribution, population viability may be reduced. Population modeling suggests spotted owl viability is sen- sitive to dispersal dynamics (Doak 1989, Lande 1991, Lamberson et al. 1992, Lamberson et al. 1994); thus, dispersal has been an important

I Present address: U.S. Fish and Wildlife Service, Oregon State Office, 2600 S. E. 98th Avenue, Suite 100, Portland, OR 97266, USA.

2 Present address: National Biological Service, 4512 McMurray Avenue, Fort Collins, CO 80525, USA. M

Present address: The Wildlife Management In- stitute, 8035 N.W. Oxbow Drive, Corvallis, OR 97330, USA.

140



J Wildl. Manage. 61(1):1997 SPOTTED OWL NATAL DISPERSAL * Miller et al. 141

component in the conservation strategies developed for spotted owls (Thomas et al. 1990, Harrison et al. 1993, U.S. Dep. Agric. et al. 1993).

To better understand the role of dispersal in populations of spotted owls, we tested 6 null hypotheses concerning the relation between habitat selection, forest fragmentation, probability of mortality, and dispersal distance: (1) dispersing juvenile owls select older-aged forests over younger-aged forests, (2) dispersing juvenile owls select equally between less fragmented forests and more fragmented forests, (3) the probability of mortality of dispersing juvenile owls in younger-aged forests is equal to that in older-aged forests, (4) the probability of mortality of dispersing juvenile owls in more fragmented forests is equal to that in less fragmented forests, (5) the probability of mortality of juvenile owls is independent of their natal dispersal distance, and (6) natal dispersal distance is independent of the amount of forest fragmentation.

We are grateful to R. L. Brandt, V. Castel- lano, S. K. Nelson, J. A. Reid, F. F. Wagner, N. T. Weiss, and W. Wright for their assistance with field work. B. Adams and D. K. Edwards provided support with the landsat analysis. Dis- cussions with L. W. Brewer, A. B. Carey, M. A. Hemstrom, R. S. Holthausen, B. G. Marcot, and L. F. Ruggierro regarding forest fragmentation analyses were helpful. D. M. Heisey and M. B. Samuel provided biometric support. J. B. Bu- chanan, D. P. DeMaster, and R. J. Gutierrez reviewed the manuscript. Staff of the U. S. For- est Service, Bureau of Land Management (BLM), and Oregon Department of Forestry provided habitat information. Funding was provided by the Old Growth Forest Wildlife Habitat Pro- gram, U. S. Forest Service, Pacific Northwest Forest and Range Experiment Station, Olympia, Washington, BLM, and Oregon Department of Fish and Wildlife.

STUDY AREAS Based on the location of nesting adult spotted

owls (Miller 1989), we selected 7 areas in western Oregon to examine natal dispersal during 1982-85 (Fig. 1). Six areas were within the Western Hemlock Zone, the most extensive vegetation zone in western Oregon and the most important for timber production (Munger 1930, Franklin and Dyrness 1973). Subclimax Doug- las-fir (Psuedotsuga menziesii) dominated this

zone, with western hemlock (Tsuga hetero- phylla) and western red cedar (Thuja plicata) the 2 other major tree species. A mild, maritime climate of relatively dry summers and wet win- ters characterized the Western Hemlock Zone, with winter precipitation falling as rain at lower elevations, especially in the Coast ranges, and as snow at higher Cascade elevations (Franklin and Dyrness 1973). These areas occurred on the Willamette National Forest (NF), Salem District BLM, Eugene District BLM, and the northern portion of Roseburg District BLM. The seventh area was in the southern portion of the Roseburg District BLM, a transition area between the Western Hemlock and Mixed Conifer zones (Franklin and Dyrness 1973). The Mixed Co- nifer Zone was generally hotter and drier during the summer months than the Western Hemlock Zone, and in addition to Douglas-fir, the predominate tree species were sugar pine (Pinus lambertiana), ponderosa pine (Pinus ponderosa), incense cedar (Libocedrus decurrens), and grand fir (Abies grandis). Clearcutting was the predominate logging method in all study areas.

Once juvenile owls dispersed from their natal forested regions in the Western Hemlock Zone, they used the 3 interior valleys of western Ore- gon: the Willamette, Umpqua, and Rogue. The vegetation within these valleys was a mosaic of grassland, oak woodland, conifer forest, ever- green shrub, and riparian communities (Frank- lin and Dyrness 1973). Urban, suburban, and agricultural areas also were present within the interior valleys.

METHODS

Telemetry We visually relocated juvenile owls for 3-4

weeks after fledging (late May through mid-Jun) until they gained sufficient weight (at or near ad wt) and could fly to elevated perches before we equipped them with radiotransmitters. Us- ing either a noose pole or long-handled dip net (Forsman 1983), we captured and then fitted owls with backpack radiotransmitter packages consisting of AVM SM-1 modules (AVM Instru- ment Co., Livermore, Calif.) and 5-mm teflon tubing for straps. The total weight of the trans- mitter packages was

<23 g (3-4% body wt). We used a Telonics TR-2 receiver and hand-held, 2-element, Yagi antenna (Telonics, Mesa, Ariz.) to relocate owls. When visual confirmation was not possible, we estimated owl locations by tri-



142 SPOTTED OWL NATAL DISPERSAL * Miller et al. J. Wildl. Manage. 61(1):1997

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Fig. 1. Study area locations for habitat selection of spotted owls during natal dispersal in western Oregon, 1982-85: (A) H. J. Andrews Experimental Forest vicinity-Cascades; (B) Eugene District BLM lands-Coast ranges; (C) Salem District BLM lands- Coast ranges; (D) Roseburg District BLM lands-Coast ranges; (E) Eugene District BLM lands-Cascades; (F) Roseburg District BLM lands-Cascades; (G) Roseburg District BLM lands-transition area.

angulation using at least 3 bearings. After the initiation of dispersal, we monitored owls daily if time and weather permitted. If dispersing owls could not be relocated by ground crews,

we used aircraft equipped with 2, 2-element, Yagi antennas to find the birds.

We considered natal dispersal to begin when an owl left its natal area (the area where it spent



J. Wildl. Manage. 61(1):1997 SPOTTED OWL NATAL DISPERSAL * Miller et al. 143

the summer) and to continue until the initiation of breeding (Greenwood 1980). Hence, juvenile spotted owls generally continue their natal dispersal until at least 2 years old (their third spring/ summer), but more commonly 3 years old (Mil- ler 1989, Wagner and Meslow, unpubl. data). In addition, we considered the dispersal process as 2 separate phases: transience, a period of extensive movement from 1 area to another, and colonization, the period when an animal attempts to become established in a new area (Johnson and Gaines 1985, Small et al. 1993). The initiation of transient dispersal was clearly detectable, because the movement of juvenile owls away from their natal areas was distinct and rapid. Cessation of transience, and thus the initiation of colonization dispersal, also was easy to detect because most owls began to roost in

relatively discrete areas for the rest of their first winter. Specifically, we considered groups of

_5 consecutive locations in an area _<2.4-km

radius to represent colonization areas. A 2.4-km radius represented an average home range size for spotted owls in western Oregon (Forsman et al. 1984). Owl locations within such areas were classified as colonization dispersal, whereas all other owl locations were classified as transient dispersal.

Habitat Classification We recognized 5 vegetation types (Hall et al.

1985) in the habitat selection analysis: (1) older forest (old-growth and mature stands), characterized by stands with an average diameter at breast height (dbh) >53.3 cm and usually <100% crown canopy closure, at least 2 height classes of trees, decay in living trees, snags, and downed

woody material; (2) closed sapling-pole-sawtim- ber stands, characterized by trees of 2.5-53.3- cm dbh and crown canopy >60%; (3) open sapling-pole stands, characterized by stands >2.5- cm dbh and crown canopy <60% (saplings were 2.5-10.2-cm dbh, poles 10.2-22.9-cm dbh); (4) clearcuts (i.e., grass/shrub) characterized by trees <2.5-cm dbh and <40% crown canopy; and (5) "other", which included reservoirs, rock out-

crops, pastures, and towns. We delineated these vegetation types by in-

terpreting aerial photographs and orthophoto- quads from each NF or BLM district forest stand information. Satellite imagery (Landsat) information was available for some parts of the Cas- cade study areas, and we used it to assist interpretation.

Habitat Use and Availability We used a circular sampling grid of 1.6-km

radius (ca 10 km2) to estimate habitat use and availability; 1.6 km represents the mean daily straight-line dispersal distance of juvenile owls during transient dispersal (Miller 1989). The average distance between successive sampling grids was about 6.6 km (4.1 x 1.6 = 6.6; 4.1 =

• no.

days between successive locations), resulting in

non-overlapping samples. The grid center was placed on each estimated owl location (see be- low), with the grid being divided into 112 0.3- x 0.3-km cells. Each cell represented the ap- proximate size (9 ha/22 acres) of an average forest harvest unit. To randomize grid sampling with respect to any landscape patterns, the grid was rotated randomly 0-45 degrees for each location and random point. We then used 50 uniformly spaced grid cells, out of the 112 cells available in the overall grid, and 50 uniformly spaced cell intersections to estimate habitat ho-

mogeneity and percentage of each vegetation type, respectively, within the 1.6-km circular grid landscape for each location.

We estimated habitat use for each transient and colonization owl location to be the percentage of each vegetation type at the 50 selected points (cell intersections) within the sample grid. Habitat homogeneity for each location was estimated by the proportion of the 50 selected cells in the sampling grid that contained 100% of 1 of the 2 closed-canopy forest types (older forest or closed sapling pole). We then indexed forest fragmentation for each 1.6-km radius circle sampling grid by combining the

homogeneity score with the proportion of older forest, (1.0 - [homogeneity score + proportion older forest]/2), resulting in a fragmentation index value between 0.0 and 1.0. Thus, low fragmentation indices were obtained for grids con- taining relatively large contiguous stands of closed canopy forests, whereas grids composed primarily of open sapling pole stands and/or fragmented by recent timber harvesting received higher fragmentation indices.

For each owl location recorded during transient dispersal, we selected 2 1.6-km radius sam-

pling grids to estimate habitat availability. The location of these grids was based on the bearing and distance between 2 consecutive owl locations, designated Oz and 02. Specifically, 2 vectors were established, each originated at 01 and set at 45 degree angles on either side of the dispersal path between O0 and 02; the length of




the vectors equaled the distance between 0, and 0, (Miller 1989). We then centered the availability sampling grids at the end of the 2 vectors, and measured the habitat characteristics (% of each vegetation type and fragmentation) as outlined above.

We selected 4 sampling grids to estimate habitat availability for each colonization area; an individual juvenile could have several colonization areas. We determined the location of these grids by first creating a polygon that circum- scribed the locations within each colonization area, then fixed 4 vectors in the 4 cardinal di- rections originating from the center of the polygon. A vector's length equaled the greater of (1) twice the distance from the polygon center to the edge of the polygon along each vector, or (2) 1.6 km from the edge of the polygon. We then calculated mean habitat characteristics from the 4 availability samples, and compared this mean to the mean from the locations within the colonization area. Sampling methodology for habitat use and availability was described in greater detail by Miller (1989).

Statistical Analyses We chose the Friedman test (Conover 1980)

to examine selection among the 5 habitats by dispersing juvenile owls. This method gives equal weight to all animals, regardless of the number of observations on each, and tests the hypothesis that ranks of differences between use and availability for each animal are the same for all habitats (Alldredge and Ratti 1986, 1992). If this hypothesis was rejected, we used the conserva- tive Tukey's multiple comparison test (Day and Quinn 1989) to determine how owls selected among the 5 habitats. To compare selection strategies for each habitat among individual owls we plotted individual selection variability (Tho- mas and Taylor 1990). We grouped fragmentation indices into 3 levels: low, >0.0 and <0.4; medium,

_0.4 and <0.7; and high, >0.7 and

<1.0. We then used the Friedman test to determine if there was selection among the 3 levels of fragmentation.

To determine if the forest structure used by juvenile owls during dispersal affected their probability of mortality, we performed regression analyses using the nonparametric Cox pro- portional hazard model (Cox 1972, Johnson 1994: 408). The number of days an owl survived after it began transient dispersal, including the num-

ber of days in colonization, was entered as the dependent variable, and interpreted as an index to the probability of mortality. Only known mortalities were treated as "failures", with un- known losses (e.g., signal loss) designated as cen- sored. Due to the small number of mortalities recorded, we assumed survival was equal among years. The percentage of each vegetation type and the fragmentation index during both transience and colonization dispersal were entered as independent variables (covariates). We also tested whether these covariates influenced dispersal distance by using the net dispersal distance as the dependent variable. Only the data from owls that survived transient dispersal during their first autumn of life were used for this test, with net distance defined as the straight- line distance between the natal area and the location of death or the location on 31 May of the following year; that date represented the general 1-year anniversary from fledging. Final regression model selection was based on results from entering each covariate individually, and both forward and backward stepwise proce- dures. The Wald test was used to determine which covariates significantly influenced the dependent variables.

RESULTS Habitat Selection

Older forest was the most frequently used habitat during both transient and colonization dispersal; yet, colonization areas occurred in older forests at nearly double their availability (61 vs. 33%) whereas the difference during transient dispersal was small (35 vs. 31%; Table 1). Habitat selection by individual owls also indicated disparate selection patterns between the 2 periods of dispersal (Fig. 2). During colonization, differences between use and availability were consistently positive and relatively greater for closed-canopy forests than the negative values observed for open-canopy forests; most differences were relatively small for all habitats during transience.

Juvenile owls selected among the 5 habitats in a similar manner during both periods of dispersal, based on the mean ranks of differences between habitat use and availability: older forest, closed sapling, "other", open sapling, and clearcut (descending order) (transience: T2 = 2.30; 4, 80 df; P < 0.06; colonization: T2 = 15.32;



J Wildl. Manage. 61(1):1997 SPOTTED OWL NATAL DISPERSAL * Miller et al. 145

Table 1. Habitat use and availability for juvenile spotted owls during transient dispersal and at roosting areas, western Oregon, 1982-85.

Transient dispersala Colonization dispersalb

Use Availability Use Availability

Habitat ?% SE 2% SE 2% SE % SE

Old-growth 35.3 16.1 31.1 16.2 61.2 33.0 33.3 18.1 Closed sapling 23.5 10.8 25.8 11.7 28.7 28.6 22.4 12.7 Open sapling 18.2 10.2 18.8 8.1 8.2 11.2 19.1 10.5 Clearcut 17.1 9.5 17.9 10.7 1.2 2.1 17.1 11.1 Otherc 5.9 8.2 6.5 8.7 1.1 2.0 7.9 12.0

a n = 21; ranks of differences between use and availability significant at P < 0.07. b n = 18; ranks of differences between use and availability significant at P < 0.001. c Pastures, reservoirs, towns.

4, 68 df; P < 0.001). The results from the sub- sequent Tukey multiple comparison tests suggested evidence of selection (P < 0.05) for 2

groups of habitats during transient dispersal. Owls selected older forests, closed saplings, and "other" habitats disproportionately more or

equal to their availability, whereas open sapling and clearcuts were used proportionately less than

expected based on availability (Fig. 2). During colonization, there was evidence of selection (P < 0.05) for 3 groups of habitats; older forests were selected disproportionately more relative to their availability, closed sapling and "other" were selected slightly less than expected, and open sapling and clearcut were selected less relative to all other habitats.

Juvenile owls did not exhibit selection among the 3 levels (low, medium, high) of fragmentation during either transience (T2 = 0.72; 2, 20

df; P = 0.51; Fig. 3) or colonization (T2 = 0.94; 2, 15 df; P = 0.43; Fig. 4). Likewise, net dispersal distance was independent of the amount of fragmentation (Wald test = 0.10, P = 0.92). Net

dispersal distance decreased with an increased use of clearcuts during transience (Wald test =

2.42; P = 0.005; Fig. 5); the use of all other habitats did not significantly (P > 0.05) affect

dispersal distance.

Mortality Only 2 habitat parameters significantly (P =

0.03) affected the probability of mortality during dispersal; an increased use of open sapling stands during transience decreased the probability of mortality (Wald test = 2.21; Fig. 6), whereas an increased use of clearcuts during colonization increased the probability of mortality (Wald test = 1.95; Fig. 7). Net dispersal distance did not affect the probability of mor-

TRANSIENT DISPERSAL 0.2

Id 0.0 - a a b

b a

0.0

-0.1

-0.2

-0.3 Z OF CS OS CC 0

Mo COLONIZATION DISPERSAL O 0.6

S 0.5- a

0.3 -

0.2- b 0.1 -

bbz o 0.0

S-0.1

-0.2 -0.3

rW -0.4

OF CS OS CC O

Fig. 2. Habitat selection variability among individual juvenile spotted owls during transient dispersal (n = 21) and colonization (n = 18), western Oregon, 1982-85. Boxes indicate the 25-75th percentile range and contain the median line, whereas capped bars represent the 10th and 90th percentile values; closed dots represent points outside these values. Lower case letters designate groups of habitats which owls selected in a similar manner relative to availability. OF = older forest; CS = closed sapling; OS = open sapling; CC = clearcut; 0 = "other" (see text).




TRANSIENT DISPERSAL 0.80

z OAVAILABILITY

S0.60 -MUSE

O 0.40 -

0.00

LOW MED HIGH

FRAGMENTATION

Fig. 3. Mean percent use and availability, based on the extent of fragmentation, during transient dispersal of juvenile spotted owls (n = 21), western Oregon, 1982-85. Error bars represent standard error.

tality (t = 0.45, P = 0.67), based on a comparison between net dispersal distance of those owls that survived the autumn phase of transient dispersal (t = 32.0 km, SD = 13.9, n = 14) and those that did not (Z = 34.8 km, SD = 21.3, n = 18).

DISCUSSION Juvenile spotted owls selected closed-canopy

forests over open-canopy forests during both the transient and colonization phases of natal dis-

persal. Our observation of a selection for older

closed-canopy forests by spotted owls during natal dispersal confirms similar preliminary results in a dispersal study conducted in California (Gu- tibrrez et al., unpubl. data), and thus strengthens the association between owls and older forests

previously established (Forsman et al. 1984, Ca-

rey et al. 1990, Ripple et al. 1991). Although owls selected closed-canopy forests over open-

COLONIZATION DISPERSAL 1.00 0.80 0 AVAILABILITY

E USE

= 0.40

0.20.......... . .. . .............

0.00- LOW MED HIGH

FRAGMENTATION

Fig. 4. Mean percent use and availability, based on the extent of fragmentation, during colonization dispersal of juvenile spotted owls (n = 17), western Oregon, 1982-85. Error bars represent standard error.

Fig. 5. Mean percent clearcut used during transient dispersal of juvenile spotted owls (n = 20) in western Oregon, 1982-85, compared to their net dispersal distance.

canopy forests during both natal dispersal periods, the magnitude of that habitat selection was not equal between the periods. Owls dem- onstrated a more distinct selection between closed- and open-canopy forests and relatively greater selection for older forests during colonization as compared to transience. Preliminary results from a study in Washington (Allen and Brewer 1985) also indicated a lack of strong habitat selection by juvenile spotted owls during dispersal.

The selection for older forests during the colonization phase of dispersal, accompanied by large individual selection variability (Fig. 2), further indicates the need for more intensive habitat use studies within these forests. Because

spotted owls remain in the colonization phase of dispersal until they breed at 2-3 years of age (Thomas et al. 1990) and, in general, subadult

spotted owls have a lower estimated fecundity than adults (Burnham et al. 1994), an increased

understanding of habitat selection within older forests would help determine if age-specific selection strategies occur and how important they are to survival. Newton (1991b) proposed that the probability of breeding by sparrowhawks (Accipter nisus) is influenced by an interaction between foraging efficiency and habitat quality (based on food supply). Newton and Marquiss (1991) reported that breeding attempts by younger (often less efficient foragers) sparrowhawks were indeed dependent on the availability of

"high quality" habitat. Thus, future habitat use studies of spotted owls within older forests should include concurrent estimates of age-specific survival and reproduction to provide an under-

standing of the processes which govern species-

80

z 60

S 40

S20"

0 0.1 0.2 0.3 0.4 0.5

PROPORTION CLEARCUT




300

250

" 200 15-- 150

S100.

0 501

0 0.1 0.2 0.3 0.4 0.5

PROPORTION OPEN SAPLING

Fig. 6. Mean percent open sapling used during transient dispersal (n = 20) of juvenile spotted owls in western Oregon, 198?-85, compared to the number of days they survived after the initiation of transient dispersal.

habitat relations (Van Horne 1986, Murphy and Noon 1991).

Dispersing juvenile owls selected equally between less fragmented forests and more fragmented forests. Although quantifying forest fragmentation is difficult (Morrison et al. 1992), indirect evidence of how fragmentation could affect dispersal comes from our observation of a negative relation between dispersal distance and the percentage of clearcut used during transience. This result suggests that the presence of clearcuts may decrease the propensity for continued transient dispersal, i.e., clearcuts may represent a barrier. Similarly, several juveniles encountering potential barriers (e.g., cities or major agricultural areas) terminated transience and settled next to the barrier (Miller 1989). Hence, although fragmentation will require owls to disperse greater distances to locate older forest patches, owls that confront clearcuts while dispersing in fragmented forests may actually reduce their dispersal distance. Dispersal distance has been a major factor in the design of conservation strategies for the spotted owl and has been one of the key components considered for spacing of both Habitat Conservation Areas outlined in the Interagency Scientific Commit- tee Report (Thomas et al. 1990), the Designated Conservation Areas outlined in the final draft Northern Spotted Owl Recovery Plan (U.S. Dep. Inter. 1992), and Late Successional Reserves as developed by the Forest Ecosystem Manage- ment Assessment Team (FEMAT; U.S. Dep. Agric. et al. 1993).

The probability of mortality of dispersing juvenile owls was affected by their habitat selec-

300

250

= 200

150

100

S50

0 0.00 0.01 0.02 0.03 0.04 0.05 0.06

PROPORTION CLEARCUT

Fig. 7. Mean percent clearcut used during colonization dispersal (n = 16) of juvenile spotted owls in western Oregon, 1982-85, compared to the number of days they survived after the initiation of transient dispersal.

tion during dispersal, as the likelihood of death decreased with increased use of open sapling stands during transience and increased with use of clearcuts during colonization. How an increased association with open sapling stands

during transience would decrease mortality is difficult to interpret. Such stands were ranked lowest (along with clearcuts) in our habitat selection analyses, and Forsman et al. (1984) re-

ported a similar low use of those habitat types for adult spotted owls. Prey availability prob- ably is a critical factor during dispersal, because many dispersing spotted owls die from starvation (Miller 1989, Gutierrez et al., unpubl. data); food supply is also critical to the survival of

Tengmalm's owls (Aegolius funereus) during natal dispersal (Korpimaiki and Lagerstrim 1988). Thus, a possible explanation for the decreased mortality associated with open sapling stands is that they could provide a relatively high source of prey compared to other habitats. The dusky-footed woodrat (Neotoma fuscipes) is the dominant prey species for spotted owls in the mesic forests of southern Oregon (Forsman et al. 1984), and attained highest densities in

"sapling/brushy poletimber stands" (similar in

vegetation structure to our open sapling stands) in northwest California (Sakai and Noon 1993). Sakai and Noon (1993) suggested that because dusky-footed woodrats will disperse from open sapling stands into adjacent older forest stands

commonly used by owls, that such sapling stands would be beneficial to owls if owls were limited

by prey availability. We suggest that dispersal may be limited by prey availability, because the majority of deaths during dispersal are from




starvation (Miller 1989). Thus, sapling stands may benefit owls by providing a source of prey that decreases mortality. We recognize that the northern flying squirrel (Glaucomys sabrinus), a species not associated with open sapling stands, replaces the woodrat as the dominant prey item in the central Cascades and Coast ranges of western Oregon. Hence, our interpretation does not suffice for those parts of our study area where flying squirrels are the predominant prey.

An increased probability of mortality with an increased use of clearcuts during colonization supports the contention that such open areas decrease successful dispersal. As only 1% of the habitat use by roosting owls was in clearcuts, this result is striking; yet, should be considered tentative given the small sample size. In a preliminary report, juvenile spotted owls in Cali- fornia that entered unsuitable habitat, including clearcuts, also appeared to suffer higher mortality (Gutierrez et al. 1985). Two factors may contribute to increased mortality in clearcuts: a greater vulnerability to avian predation by great horned owls (Bubo virginianus) and northern goshawks (Accipiter gentilis) as suggested by Dawson et al. (1987); and decreased foraging activity, resulting in starvation, following the termination of dispersal at an abrupt habitat edge (Miller 1989; E. Forsman, U. S. For. Serv., pers. commun.).

Obtaining information on dispersal dynamics useful in developing management and conservation strategies remains a fundamental chal- lenge. Defining habitat availability during dispersal was essential to our research, and we recognize our conclusions depend upon that defi- nition (Johnson 1980). Specifically, Johnson (1980) discussed the concept of a hierarchy of habitat selection where the order of selection is dependent on previous selection processes, e.g., selection of (1) geographical range, (2) home range, and (3) habitat components within the home range. Spotted owls select habitats at all 3 orders during dispersal, thus confounding the concept of availability. We defined habitat availability during dispersal from the estimated dispersal paths of individual owls, as opposed, for example, to all habitats available in a larger area surrounding the nest site (natal area). Hence our inferences are based on the assumption that owls already displayed some level of selection. Our estimate of older forests as the most abundant available habitat demonstrates such prior selection by dispersing juvenile owls, because

these forests do not comprise the most abundant habitat in the overall study areas.

MANAGEMENT IMPLICATIONS Natural resource managers require an un-

derstanding of the effect of habitat-management activities on the viability of wildlife populations (Marcot 1986). Our research indicates that spotted owls selected older closed-canopy forests during natal dispersal, reinforcing the strong association between this species and such forests. Thus, the question of how the continued loss and fragmentation of older closed-canopy forests will affect the viability of owl populations depends, in part, on the distribution of remain- ing forests relative to the owls' ability to locate them (McLellan et al. 1986, Pulliam and Dan- ielson 1991). Morrison et al. (1992:87) recom- mended that patches of rare habitat be acces- sible to species that demonstrate strong habitat selection, such as the spotted owl. Maintaining some older forest in the matrix surrounding the patches occupied by breeding adults will provide colonization areas for subadults during the years before their recruitment into the breeding population, and potential travel corridors for both juvenile and displaced adult owls during transient dispersal. Selection of older closed-

canopy forests during dispersal supports the concept of maintaining adequate dispersal habitat (through a variety of means) among designated reserve areas as described in the Interagency Scientific Report (Thomas et al. 1990), final draft Northern Spotted Owl Recovery Plan (U.S. Dep. Inter. 1992), and the FEMAT Report (U.S. Dep. Agric. et al. 1993). Alternative management strategies such as the draft Preservation Plan for the Northern Spotted Owl (Lujan et al. 1992) would not be supported by our results because adequate dispersal conditions would not be maintained over a large portion of the species' range and important connections between phys- iographic provinces would be lost. Because our results indicate that dispersal distances decline as juvenile owls disperse through clearcuts, and use of clearcuts may increase their mortality, we recommend a reduction in the number of clearcuts to increase successful dispersal. Such a reduction in clearcuts not only should facilitate more successful transient dispersal, but provide for some level of older forest being maintained throughout the matrix for colonization by young owls. Additional information on the dynamics of owl dispersal, particularly habitat-specific




survival rates, is needed to develop better management and conservation strategies.

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Received 13 January 1995. Accepted 17 April 1996. Associate Editor: Ryan.



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