Ecology of the Cooper's Hawk in North Florida

Abstract

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.

Male brood provisioning rates provide evidence for inter‐age competition for mates in female Cooper's Hawks Accipiter cooperii

“…Similar age‐related differences in reproductive success have been reported for Cooper's Hawks in Arizona and Florida (Boal , Millsap et al . ). It is possible that SY females have a lower innate reproductive potential and thus are physiologically constrained to produce smaller clutches of eggs.…”

“…These findings support the hypothesis that intrasexual competition for mates and nesting territories does occur among female accipiters (Newton , Olsen & Olsen , Kenward , Millsap et al . ), and that ASY females are most likely better competitors for high‐quality mates/territories.…”

“…In the Northern Goshawk Accipiter gentilis , Eurasian Sparrowhawk Accipiter nisus and Cooper's Hawk Accipiter cooperii , males appear to compete for breeding territories prior to the breeding season, whereas females compete for high‐quality males/territories (Newton , Kenward , Millsap et al . ). Males in these species do nearly all the hunting for the female and the brood, exclusively so in many cases (Rosenfield & Bielefeldt , Eldegard et al .…”

“…, Millsap et al . ). Territories with higher prey densities facilitate higher rates of male prey delivery, implying that territory quality varies with the abundance of prey (Snyder & Snyder , Penteriani et al .…”

“…Prior to reproduction, many female Cooper's Hawks visit several prospective males, presumably assessing their provisioning ability, and then compete for desirable mates (Millsap et al . ). High‐quality males are a critical resource for increasing female reproductive success.…”