Home Range and Habitat Use by American Crows in...

Home Range and Habitat Use by American Crows in Relation to Transmission of West NileVirusAuthor(s): Sarah A. Yaremych, Robert J. Novak, Arlo J. Raim, Phil C. Mankin, Richard E.WarnerReviewed work(s):Source: The Wilson Bulletin, Vol. 116, No. 3 (Sep., 2004), pp. 232-239Published by: Wilson Ornithological SocietyStable URL: http://www.jstor.org/stable/4164671 .Accessed: 17/02/2012 22:22

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Wilson Bulletin 1 16(3):232--239. 2004

HOME RANGE AND HABITAT USE BY AMERICAN CROWS IN RELATION TO TRANSMISSION OF WEST NILE VIRUS

SARAH A. YAREMYCH,'34 ROBERT J. NOVAK,2 ARLO J. RAIM,1 PHIL C. MANKIN,' AND RICHARD E. WARNER'

ABSTRACT.-Determining the sizes and characteristics of home ranges among American Crows (Corvus brachyrhynchos) in spring/summer is essential for understanding the importance of this species in the transmission of West Nile virus. In late spring, summer, and early fall of 2002, we radio-tracked 45 American Crows to estimate their movements and habitat preferences in urban and adjacent agricultural habitats in east-central Illinois. The 95% minimum convex polygon home ranges averaged 9.6, 6.7, and 6.4 km2 for hatch-year, sub- adult, and adult crows, respectively. Proportional habitat use was investigated at two scales: (a) at the study site scale, there was a preference for low- to medium-density urban habitat and avoidance of forested habitat; and (b) at the home range scale, there was a preference for agricultural cover and avoidance of high-density urban habitat. Received 16 October 2003, accepted 19 July 2004.

Corvid populations are increasing world- wide in response to urbanization (Marzluff et al. 2001), and the American Crow (Corvus brachyrhynchos) is a successful exploiter of human environments in North America. Home-range estimates for crows are distinct in different geographic settings, and range from 0.01 km2 in highly urban environments to over 35 km2 in non-urban landscapes (Mar- zluff et al. 2001). A number of studies (re- viewed by Verbeek and Caffrey 2002) have contributed to our understanding of spatial and geographic area requirements of crows. During winter, Stouffer and Caccamise (1991) captured crows on agricultural fields in New Jersey and divided them into two groups: diurnal, activity-center based (DAC) crows and vagrant crows. DAC crows used daytime territories ranging in size from 0.3 to 0.5 km2 and returned to the communal feeding area nearly every day. Vagrants could be found during only 31.8% of daytime tracking at- tempts; crows of both groups roosted together at night. McGowan (2001) found that suburban crows in New York defended 0.09 km2 territories, whereas their rural counterparts de-

fended 0.4 km2 territories in the spring. Sul- livan and Dinsmore (1992) determined that, during the breeding season, home-range size for 10 pairs of crows in Manitoba was 2.6 km2.

The prevalence of vector-borne disease is often related to the home-range size, habitat availability, and habitat selection of host species. The American Crow is a reservoir host for the mosquito-transmitted West Nile virus (WNV; McLean et al. 2001). The birds attain a high blood viral titer before death (McLean et al. 2001) and may serve as amplifying hosts, increasing local transmission. Dead, WNV-positive crows may serve as effective sentinels of local transmission (Nasci et al. 2002, Guptill et al. 2003). To date, there has been little study of movement and habitat use by crows in urban areas during the period when the risk of transmitting WNV is high. Knowledge of home-range size, the geographic area over which an infected crow may po- tentially move, and habitat composition are essential for understanding how crows relate to transmission foci for WNV.

Our study of crows in a Midwestern urban- ized setting with adjacent agricultural lands was designed to (1) estimate home-range sizes of crows during the late spring through early fall season, (2) describe habitat use, and (3) relate our findings to the potential for WNV transmission by this species.

METHODS

From February through October 2002, we captured crows on communal feeding grounds

' Dept. of Natural Resources and Environmental Sciences, Univ. of Illinois, W-503 Turner Hall, 1102 S. Goodwin, Urbana, IL 61801, USA.

2 Center for Economic Entomology, Illinois Natural History Survey, 607 E. Peabody Dr., Champaign, IL 61820, USA.

I Current address: 13 Natural Resources Bldg., Fish- eries and Wildlife Dept., Michigan State Univ., East Lansing, MI 48824, USA.

4 Corresponding author; e-mail: [email protected]

232

Yaremych et al. * HOME RANGES OF AMERICAN CROWS 233

with Australian crow traps (Kalmbach and Al- dous 1940, Rowley 1968) within 1.6 km of the South Farms agricultural property (400 2' N, 880 17' W) of the University of Illinois in Urbana-Champaign, Illinois. Crows were aged by palate coloration and plumage characteristics (Pyle 1997) as hatch-year, sub-adult (year-old individual), or adult (second year and older). We weighed, measured, and band- ed birds at capture, and determined their gender by discriminant analysis of external mea- surements. Using a sample of known-sex individuals, we used age-group-specific discriminant functions to determine gender. A series of eight mensural characters were evaluated for use in discriminant analysis, including bill length, width, and depth; head-to-bill length; length of wing chord, tail, and tarsus; and mass. The performance of each function was evaluated through cross validation, yielding 94, 100, and 78% correct classification for hatch-year, sub-adult, and adult crows, respectively (Yaremych et al. 2004a).

Radio-transmitters were attached to a sub- sample (n = 56) of captured crows. Radio- transmitters were custom designed (by W. Cochran, Champaign, Illinois), and weighed <2 g for tail-mount, and <3 g for collar- mount. Transmitters had an expected battery life of approximately 6 months. We used a tail-mount attachment most frequently (n = 46), modifying the method of application of Dunstan (1973). Attachment was to the rachis of the central tail feather. The back of the transmitter and battery were lined with epoxy and laid on top of the central tail feather shaft, with the transmitter roughly 2.5 cm below the uropygial gland. We used a needle and thread to stitch through the rachis under the transmitter, and wrapped thread repeatedly around the transmitter and battery and under the tail feather. Five additional stitches were used to secure the antenna to the rachis; the antenna ended at the distal tip of the tail feather. All sutures were coated with epoxy to protect them from weathering. Alternatively, collar- mount transmitters were attached to crows caught before molting (n = 10). Collars were individually adjusted for optimal sizing and sewn around the necks of crows.

Each radio-tagged crow was tracked using vehicle-mounted twin Yagi antennas to obtain locations approximately 10 times per week

(Cochran 1980). Tracking was stratified equal- ly between morning (06:00-12:00 CST), afternoon (12:00-20:00), and nighttime (20:00- 06:00). We searched for birds that we were unable to locate while on foot or by vehicle with fixed-wing aircraft. Point locations, taken with Global Positioning Systems (GPS; Gar- min GPSIII), were used for analysis of home range. All GPS locations were recorded while crows were on the ground or in trees. Typi- cally, the minimum number of radio-locations required to yield a stable home-range size was 10, and home-range size was not estimated for crows that were located <10 times. Radio- tracking efforts for the remaining crows de- clined after 31 October.

Any crow that was encountered dead was tested for West Nile virus using immunohis- tochemical staining (Heinz-Taheny et al. 2004) at the University of Illinois College of Veterinary Medicine, or TaqMan reverse transcriptase-polymerase chain reaction (Lanciotti et al. 2000) at the Medical Entomology laboratory at the Illinois Natural History Survey.

Locations were plotted on the 1999-2000 satellite imagery of Illinois Land Cover (Illi- nois Interagency Landscape Classification Project 2003) using a Geographical Informa- tion System (Environmental Systems Re- search Institute 1998). The 95% minimum convex polygon (MCP) estimator for home range was used to describe the outer limits of each crow's movement (Mohr 1947). Addi- tionally, adaptive kernel (ADK) home ranges were computed at the 50 and 95% contour levels (Worton 1989) for identification of activity centers.

We defined the study site as the cumulative area used by all the radio-tracked crows in spring through early fall; this area is the merged 95% MCP home ranges for the 45 crows included in the study. The study site, totaling 39,471 ha, encompassed the cities of Champaign and Urbana, as well as the sur- rounding agricultural areas. We delineated and categorized the cover types on the site as (a) agriculture-primarily corn, soybeans, winter wheat, and rural grassland and residences; (b) forest-areas predominantly covered with trees and woody vegetation, including upland deciduous closed-canopy forests, upland open-canopy/savannah forests, and closed- canopy floodplain forests situated in lowlands

234 THE WILSON BULLETIN * Vol. 116, No. 3, September 2004

5

4

0 03

0

E zi

0- 3/25 4/25 5/25 6/25 7/25 8/25 9/25 10/25

Dates of crow captures

FIG. 1. Capture dates of 45 American Crows used for analysis of home range and habitat use in east-central Illinois, late spring through fall, 2002.

and temporarily flooded floodplains; (c) high- density urban-all or nearly all of the land surface covered with manmade structures; (d) low- to medium-density urban up to 50% of the land surface covered with manmade structures; and (e) urban open space-including parks, golf courses, cemeteries, and other grassland-like cover within urban and built-up areas intermixed with other cover, such as forest. Each telemetry location was buffered (25 m) to account for telemetry error.

Habitat use was interpreted by compositional analysis (Aebischer et al. 1993) at two scales of analysis using Resource Selection software (Leban 1999). Compositional analysis examines habitat use, with each animal, not each location, serving as the sampling unit, thereby avoiding potential autocorrela- tion problems often associated with radio-telemetry studies (Aebischer et al. 1993). First, the relative amount of habitat within each crow's 95% MCP home range was compared to the relative amount of each habitat available within the total study site for analysis at the study site level. Second, the habitat of telemetry locations (the habitat a crow was using when we recorded a telemetry location) was compared with the relative amounts of each habitat within an individual's MCP home range for analysis at the home-range level.

The values (%) representing habitat use and availability were log-ratio transformed as part of the analysis. Resource Selection software replaced the 0% value for those habitat types that were not utilized, yet available, with

0.001% to allow for this transformation. Where there is significant nonrandom use (i.e., habitat use within the site or home range differed significantly from use at random, as determined with Resource Selection), the habitat types can be ranked in order of increasing relative use. If a habitat type was used more than expected based on its availability, it was con- sidered to be preferred; habitats were ranked from least preferred (0) to most preferred (4; Aebischer et al. 1993). Selection index graphs were created based on pairwise comparisons of means of the log-ratios of habitat use.

RESULTS

From February through October 2002, we captured 156 crows, 56 of which were fitted with a radio-transmitter. Due to death, missing signals, or radio-transmitter failure, only 45 of the radio-tagged crows remained in the study long enough for us to acquire a sufficient number of radio-locations to achieve stability in home-range estimates. Captures of these 45 crows took place from 25 March to 10 Octo- ber, with 80% of the captures occurring after mid-June (Fig. 1). This sample was composed of individuals from the various age and sex categories as follows: 26 hatch-year females, 7 hatch-year males, 3 sub-adult females, 4 sub-adult males, 4 adult females, and 1 adult male. The number of locations obtained for all crows was 2,147, with an average number of 48 locations per crow.

During our study, 68% of radio-tracked crows died and tested positive for WNV (in-


cluding some crows for which home ranges were not estimated due to a lack of sufficient number of locations; Yaremych et al. 2004b). Observations of 10 infected crows 1 to 2 days before death indicated a change in their normal movement patterns. For example, three sick crows either did not return to a nighttime roost from the communal feeding area in the afternoon, or did not fly to the feeding area from the roost in the morning. Instead, these crows were found alone between the day and nighttime areas, and they did not feed. One crow was perched in a tree with one eye closed. Other crows were found on the ground and did not move as we approached, and were found dead later that day or the next morning. We removed the last-location data points (crows not moving) before computing home ranges for the crows that we tracked imme- diately before death due to WNV. According- ly, our analysis represents the movement and habitat use of crows displaying normal movement patterns. In many situations, a crow was located at a regular roost at night, and by the next tracking session (1-2 days later), the crow was found dead in urban habitat near the roost; thus, there were no observations indi- cating that the crow was sick.

The 95% minimum convex polygon home- range sizes for hatch-year, sub-adult, and adult crows averaged 9.6, 6.7, and 6.4 km2, respectively. The 50% (core area) and 95% adaptive kernel home-range sizes were hatch-year crows: 2.3 and 14.4 km2; sub-adults: 1.1 and 6.9 km2; and adults: 1.1 and 6.1 kM2, respectively. Home-range size did not differ significantly between sexes (Mann-Whitney U-test = 154, df = 1, P = 0.44), or among age classes (Kruskal-Wallis H-test = 2.47, df = 2, P = 0.25). The average minimum convex polygon home-range size across sexes and age classes was 7.6 km2. Home ranges of crows overlapped extensively. Crows were frequently located during the day in agricultural habitat south of the residential area where they roosted. The agricultural habitat was a communal feeding area, and crows of many family groups fed there for the duration of spring-to- fall tracking.

The study site was composed of 38% agriculture, 30% low- to medium-density urban, 22% high-density urban, 5% forest, and 5% urban open space. Habitat use within the study

site (all ages and sex classes) differed significantly from random (P < 0.001); i.e., crows established home ranges in a non-random manner. Habitat use within home ranges also differed from random (P < 0.001). In the study site analysis (home range versus study site), crows preferred low-density urban areas and tended to avoid the more forested areas; in the home-range analysis (locations versus home range), crows preferred agricultural cover and tended to avoid high-density urban areas (Fig. 2). Resource ranking matrices, which rank the cover types according to relative use (highest-ranking habitat is most preferred) were (1) study site analysis (P < 0.001): forest = 0, agriculture = 1, high-density urban = 2, urban open space = 3, low-density urban = 4; (2) home-range analysis (P < 0.001): high-density urban = 0, urban open space- 1, forest = 2, low-density urban = 3, agriculture 4. Crows of all age classes dem- onstrated a general pattern of foraging in agricultural areas during the daytime and roosting in urban areas at night. All crows in this study were captured in the agricultural areas and all roosted in the adjacent urban areas, with fidelity to one or two preferred roosts and occasional travel to other urban roosts.

When comparing age classes for habitat composition of home ranges, adults used more forest (13.0%) than both hatch-year crows (5.5%; U = 145, df = 1, P = 0.004) and sub- adult crows (6.0%; U = 28.5, df = 1, P = 0.044). For each of the five habitats, no differences (U = 160-254, df = 1, P = 0.079- 0.488) were detected in habitat composition of the home ranges used by males and females. Habitat composition of telemetry locations was different among age classes for low- to medium-density urban habitat only (H = 6.58, df = 2, P = 0.038): sub-adults used this habitat more than hatch-year crows (U = 181.5, df = 1, P = 0.010) or adult crows (U = 6, df = 1, P = 0.037). Habitat composition at telemetry location sites differed between sexes: females used agriculture (U = 102, df = 1, P = 0.007) and forest (U = 130, df = 1, P = 0.042) more than males, and males used high- density urban habitat (U = 268.5, df = 1, P = 0.036) and low- to medium-density urban habitat more (U = 304, df = 1, P = 0.003) than females.


Home range versus study site

3 -

2

0 - _

-2 -

Agriculture Forest Low- to medium- High-density Urban open -3 density urban urban space

Telemetry locations versus home range

1.5

0.5

0

-0.5

-1

-1.5

Agriculture Forest Low-to medium- High-density Urban open -2 density urban urban space

FIG. 2. Habitat selection indices at the scales of study site and home range for American Crows in east- central Illinois, late spring through fall, 2002. Positive values indicate selection, and negative values indicate avoidance. Bars show log-ratio means of habitat selection for crows. Error bars are ? SE.

DISCUSSION Our data afford a general picture of where

crows spend time during late spring through fall, and are of timely interest in defining the potential role of crows in the spread of WNV. After beginning to feed on their own, young crows appeared to enter the traps on the com-

munal feeding areas more readily than the older crows, perhaps because younger birds are less experienced. The result is an age bias in our sample toward hatch-year crows. Home ranges were constructed for the cumulative season of late March through the end of Oc- tober, with most crows entering the study in


mid-June and later, and crows exiting the study throughout. The majority of the captures before mid-June were sub-adult and adult crows, whereas the majority of captures in late June and later were hatch-year birds. The differences in habitat use among age classes may relate to the temporal bias in our sampling, with adults possibly using more forest in the early portion of the study because many nest- ed in forest. As a majority of crows were captured in mid-June and later, our study does not focus on home ranges of breeding crows. In east-central Illinois, crows mate in late Feb- ruary and early March, and a majority of young crows have left the nest by May (Gra- ber et al. 1987). The crows of Urbana-Cham- paign had larger home ranges compared with those of crows studied in New Jersey, New York, and Manitoba (Stouffer and Caccamise 1991, Sullivan and Dinsmore 1992, McGowan 2001). Some of the variation among studies is due to differences in how area use was measured (defended territories in New York; com- plete 95% home ranges in Urbana-Cham- paign; Marzluff et al. 2001), seasonality (mid- April through mid-July in Manitoba; locations taken through the fall in Urbana-Champaign), and time periods (daytime home ranges in New Jersey; combined daytime plus nighttime home ranges in Urbana-Champaign).

That home-range sizes of crows did not differ among age classes may be a result of small sample sizes for the older age classes in the late season. As several crows entered the study in September and October, location data contributed by these birds to the overall home- range and habitat-use estimates were for the later months only. Accordingly, individuals captured early in the study, but which died before the study ended, contributed only early season data. We note that the general movement patterns of the crows did not differ throughout the study period: in general, all crows were captured in agricultural feeding areas, these feeding areas were used throughout the entire tracking season, most crows roosted in urban areas, and the same urban roosts were used continually throughout the tracking season.

Crows selected low- to medium-density urban cover and tended to avoid the more forested areas. Differential habitat use may be related to diet (Marzluff et al. 2001). The ur-

ban environment may provide food sources to supplement agricultural foraging, resulting in a diminishing need for forest food resources. The juxtaposition of urban and agricultural habitat within our study site may facilitate the exploitation of abundant and easily located anthropogenic food resources in the agricultural and urban areas. While in the agricultural setting, crows tended to be found feeding in the fields that were most recently tilled or har- vested to exploit recently uncovered inverte- brates and waste grains. While in the urban setting, crows commonly fed from dumpsters and were found in picnic areas and parking lots. The preference of crows for low- to medium-density urban cover may expose crows to the primary vector of WNV in Illinois, the urban-dwelling Culex pipiens (Foster and Walker 2002).

The period of our study on crow movement and habitat use overlapped the mosquito season, thus representing the WNV transmission season. The high mobility of crows under- scores the potential importance of this species in the transmission of WNV over a large spatial area, although the movement of infected crows in nature is difficult to study because the exact time of exposure of crows to WNV is unknown. The general daily pattern of feeding in agricultural cover and roosting in urban cover is evidence for potential movement of WNV between two distinct habitats if crows carry the virus between habitats and if mosquitoes feed upon birds in both areas.

In laboratory settings, crows die of WNV within 4-7 days post-infection (McLean et al. 2001, Komar et al. 2003), and are sufficiently viremic to infect a feeding mosquito for 3-5 days before death (Komar et al. 2003). Our field observations of sedentary, defenseless behavior in WNV-positive crows 1-2 days before death suggest that an infected crow may provide an easy blood meal for bird-feeding mosquitoes, and crows may serve as reser- voirs for WNV transmission. Our observations that crows became notably disabled only for a short time (reduced movement 1-2 days before death) suggest that crows may travel to regular communal feeding areas and roosts while infected and transport WNV across an average home-range size of 7.6 km2.


ACKNOWLEDGMENTS

The authors would like to thank G. L. Hamer, A. J. Arnold, and C. Warner for field assistance; J. M. Sieg- rist for software assistance; and N. M. Krasavin, R. L. Lampman, and A. M. Ringia for assistance throughout. We thank J. D. Brawn, C. Caffrey, N. Komar, and two anonymous reviewers for their significant contribu- tions to our manuscript. This research was supported by Centers for Disease Control and Prevention Grant PHS U50 CCU5205 1 (RJN), Illinois Waste Tire Fund (RJN), Illinois Department of Natural Resources, Illi- nois Natural History Survey, and the McIntire-Stennis Forestry Research Act Project (PCM and REW).

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