Ecology of the Cooper's Hawk in North Florida

We studied adult Cooper's hawks Accipiter cooperii on two study areas in north Florida from 1995 to 2001, an area dominated by large plantations managed for northern bobwhite Colinus virginianus and an area of mixed farmland and woods with no direct bobwhite management. We monitored 76 Cooper's hawk nesting attempts at 31 discrete nest areas, and radio-tagged 19 breeding males and 30 breeding females that we radio-tracked for up to 5 y. Nesting density (565 to 1,494 ha per occupied nest area) was comparable but productivity (1.8 and 2.8 young fledged per occupied and successful nest area, respectively) was lower than for the species elsewhere. Prey may have been more limiting than in other areas studied because chipmunks Tamias striatus, an important prey elsewhere, were absent. Annual Cooper's hawk survival averaged 84% for males and 81% for females, except in 1998 when survival was substantially lower. Average annual home-range size for male Cooper's hawks was 15.3 km2 inclusive of one nesting area. Female annual ranges averaged 30.3 km2, and included from three to nine nesting areas. Daily space use was similar between the sexes, but females had separate breeding and nonbreeding ranges whereas males were sedentary. Females used the same nonbreeding areas among years, but switched nesting areas 68% of the time compared with only 17% for males. Birds comprised 88% of the breeding and 98% of the nonbreeding season diet of Cooper's hawks by frequency. Important prey species all year were mourning doves Zenaida macroura, blue jays Cyanocitta cristata, and northern bobwhite; during summer, cattle egrets Bubulcus ibis, northern mockingbirds Mimus polyglottos and northern cardinals Cardinalis cardinalis were also important; and during autumn and winter, killdeer Charadrius vociferus, yellow-billed cuckoos Coccyzus americanus, and chickens were important. Female Cooper's hawks took larger prey than males; females were responsible for most cattle egret and chicken kills; whereas, males took most blue jays, killdeer, northern mockingbirds, and northern cardinals. Of avian prey brought to nests, 64% were nestling birds. Most adult male Cooper's hawks were adept at raiding bird nest boxes. Male Cooper's hawks captured 85% of the prey fed to nestlings. Female Cooper's hawks relied on males for food from early March until young were ≥12 d old, and 6 of 10 breeding females monitored intensively were never observed foraging for their broods. Most prey brought to nestling Cooper's hawks was captured within 2 km of nests, and foraging effort was consistent throughout the day. During the nonbreeding season, most prey captures occurred before 0900 hours or at dusk. Northern bobwhite made up 2% of male and 6% of female Cooper's hawk prey annually by frequency; this extrapolated to 18 bobwhite/year/adult Cooper's hawk on both study areas, 59% of which were captured between November and February. Outside the breeding season, male Cooper's hawks foraged evenly over their home range whereas females tended to focus on prey concentrations. Because female Cooper's hawks were so adept at finding and exploiting prey hotspots, perhaps the best strategy for reducing predation on bobwhite is habitat management that produces an even distribution of bobwhite across the landscape.

How is model selection determined in a vocal mimic?: Tests of five hypotheses

Many animal species imitate the sounds of other species, but we know little about why vocal mimics copy some species while failing to copy other species, i.e., ‘model selection’. In this observational study of free-living northern mockingbirds (Mimus polyglottos), I tested five hypotheses of model selection: (1) Proximity hypothesis: preferential imitation of species found in close proximity to the vocal mimic, (2) Aggression hypothesis: preferential imitation of species with which the mimic interacts aggressively, (3) Passive sampling hypothesis: preferential imitation of species heard frequently by the mimic, (4) Acoustic similarity hypothesis: preferential imitation of species whose sounds are acoustically similar to the non-imitative songs of the vocal mimic and (5) Alarm hypothesis: preferential imitation of alarm-associated vocalisations. The data supported only the acoustic similarity hypothesis. Given that this hypothesis has been supported in two additional mimicking lineages, it suggests a potential non-adaptive explanation for the evolution of vocal mimicry. Species that learn vocalisations are already predisposed toward learning sounds with key acoustic characteristics. Whenever natural selection favours a widening of the auditory template that guides model selection, vocal imitation of heterospecifics becomes more likely because of ‘learning mistakes’.