Cooper's Hawk

Accipiter cooperii

Order:
Accipitriformes
Family:
Accipitridae
Sections

Demography and Populations

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Figure 4. Relative abundance of Cooper's Hawk during the breeding season.

Based on data from the North American Breeding Bird Survey, 2011–2015. See Sauer et al. (2017) for details.

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Figure 10. Regional trends in Cooper's Hawk breeding populations.

Based on data from the North American Breeding Bird Survey, 1966–2015 (Sauer et al. 2017). Data show estimates of annual population change over the range of the survey; areas of increase are shown in blue and declines are shown in red. See Sauer et al. (2017) for details.

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Figure 11. Christmas Bird Count data 1985-2005, Cooper's Hawk.

U.S. Christmas Bird Count (CBC) data for the Cooper's Hawk, showing increase in numbers from 1985-2005.

Measures of Breeding Activity

Age at First Breeding

Usually at 2 years of age, but yearling (1-yr-old) immatures reported as 6% to 22% of breeding females in short-term studies of 3–6 years (39, 222, 212, 208, 74, 207, 183, 203); in long-term research (17–26 years) yearling females in large or metropolitan cities (e.g., Milwaukee, Wisconsin, Tucson, Arizona, Victoria, British Columbia) bred every year and averaged 16–25% of all females (but 50% in some years in British Columbia; 1). Yearling females not present each year and averaged 8% of breeding females across 39 years in both urban (Stevens Point, small city of about 35,000 people) and rural Wisconsin landscapes (1, 185). Stout et al. (185) reported a significant decline (from about 60% to 21%) in proportion of breeding yearling females as numbers of older nesting females increased across 12-year study (1993–2004) suggesting a relatively young, growing population in Milwaukee, Wisconsin.

Breeding by yearling males is nonexistent, rare, or uncommon. No breeding of yearling males detected in multi-year (maximum of 6 years) studies of Cooper’s Hawks in Florida (B. A. Millsap, personal communication), Oregon (212, 36), Arizona (208), Utah (222), California (183), North Dakota (186, T. G. Driscoll, personal communication), and Minnesota (T. G. Driscoll, personal communication). Meng (39) reported no nesting by immature males in rural New York, but first nests (n = 2) in New York City since 1955 were occupied by yearling males (133). Boal (203) reported yearling males made up 7% of all nesting males (n = 184) in Tucson, Arizona, but no break-down by year. In 17-year study, only 1 (0.2%) yearling male detected among 579 breeding pairs in Victoria, British Columbia (A. C. Stewart, personal communication). Six of 123 (4.8%) aged males in Milwaukee, Wisconsin, were yearlings (185), and in a 5-year study in Albuquerque, New Mexico, 3 (1%) of 305 breeding males were immature birds (67). None of these studies reported significant temporal trend in incidence of immature breeding males.

In a 32-year (1980–2011) study in Wisconsin, only 13 (2%) of 732 aged-breeding males were yearlings; but immature males made up a maximum of 13% of all breeding males in any year when at least one yearling male was detected (mean 6%; 205). Temporal frequency of breeding yearling males differed statistically as 12 (92%) and one (8%) of total 13 yearling males were detected in first and second 16 years of study, respectively; researchers suggested that yearlings could breed earlier in study when fewer numbers of older, competitively experienced males were present in this growing (recovering) population. Yearling males had smaller average brood counts vs older males, and yearlings had shorter maximum longevity (hence smaller lifetime reproductive output versus males that delayed breeding until older) as deleterious cost of breeding early (205). Greater extent of flight-feather molt in yearlings vs older nesting males likely aggravates costs of breeding when only 1-year-old (205, 1).

In Tucson, Arizona, pairs assorted by age: 7 (78%) of 9 yearling males were paired with immature females (203), whereas 12 (92%) of 13 yearling males in central and southern Wisconsin were paired with females at least 2-yr-old (205). A pair of subadults successfully nested in Indiana (114) and New York (234).

Clutch

See Appendix 1; also 24, 1, 235). Mean clutch size 3.3 to 4.4 eggs (range 1–8 eggs), with no discernable geographic or temporal trend (1, 235). Few long-term data; researchers generally do not count eggs (1), but 36-year Wisconsin study (1980–2015) showed no trend in average clutch counts, which ranged from 3.9 to 4.8 with an overall average of 4.3 eggs (206). Mean clutch size generally 1–3 eggs smaller in yearling than adult females (207, 236). In concurrent study years (1995–2001) average clutch size about 1 egg lower in western and north-central North Dakota versus averages in Wisconsin and British Columbia (237). For males ≥ 2 years of age in Wisconsin, no significant differences between younger and older males in mate's clutch size (18).

Annual and Lifetime Reproductive Success

See Table 3 for mean measures of annual success—values within same range in other reports (39, 224, 238, 207, 239, 152) but longer studies show high yearly variation in number of young per active nest (240). In 39-year study (1980–2018) in Wisconsin, such variation arose mainly from annual differences in nest success, with range of 57–95% (141), comparable to extremes of Table 3.

Hatching success and nest success lower in yearling than adult females in Arizona (208). In comparisons among adult, mixed (adults and subadult) and subadult pairs in Arizona, adult pairs bred earlier and raised larger broods than mixed pairs. Subadult pairs bred latest and were least productive (203).

Historical comparisons only for clutch size (unchanged) and number of bandable young per successful nest in northeastern U.S.: 3.5 bandable young in 1929–1945 (pre-DDT), 3.1 in 1946–1948, and 2.7 in 1949–1967 (DDT era; 128), but 3.3 in 1967–1976 (post-DDT in U.S.; 241).

Rosenfield et al. (113, 242) found that urban-nesting Cooper's Hawks in Wisconsin lay, on average, similar size clutches as found in rural areas and that the number of bandable young in both habitats were among the highest recorded for the species. They also found no significant differences in clutch size, number of bandable young per nest and nest success in comparisons between nests in conifer plantations and those in non-plantation habitats (107). Comparisons between nesting success in pine plantations and oak-hickory forests in Missouri revealed no differences in clutch size, brood size, or number of young per successful nest (243). Overall, these studies suggest that developed areas and conifer plantations often provide equally suitable breeding habitat for the species than do more natural areas.

In North Dakota, nest success differed significantly between woodland types represented in the study area (213); nests in upland woodlands were more successful at producing young (86% of 28 nests) than those in narrow forest patches along rivers (57% of 44 nests), a difference apparently due to more abundant predators in the riverine forests.

In Wisconsin, lifetime reproductive output averaged 8.8 and 8.7 bandable young per successful nest for 70 males who began breeding when at least 2 years of age (maximum 32 young for oldest individual, a 9-yr-old), and for 8 males who started breeding as yearlings (maximum 23 young for oldest bird, a 6-yr-old), respectively (244, 205). Lifetime production correlated with longevity, but not with habitat type or body mass, or size (244).

No lifetime productivity data for females.

Number of Broods Normally Reared per Season

Single brood per season; no records of more than 1 brood annually.

Proportion of Total Females that Rear at Least One Brood to Nest-Leaving or Independence

Not known.

Life Span and Survivorship

Oldest age of breeding bird is a 12-yr-old female in British Columbia, and the longevity record is 20 yr for migratory male in western North America (1, U.S. Geological Survey Bird Banding Lab data). Mean age at death 16.3 mo for 136 banded birds (245). Mortality rates (1941–1957) estimated as 72–78% in first year, 34–37% thereafter by life table methods (128; but see 246). Survival rate was 75% and 64% for juvenile males and females, respectively, from hatching through 180 d in Tucson, Arizona (112).

Annual survival rates of breeding birds relatively invariant across continental range: 69% and 80% for both sexes when birds in second year of life and older, respectively, in Tucson, Arizona (112); 80% rate for both sexes in rural Florida (46), and 82–88% for both sexes in Albuquerque, New Mexico (67). In only comparative, urban versus rural habitat study, non-significant difference between 84% and 79% annual survival rates across 26 yr (1980–2005) for breeding adult males in Stevens Point, Wisconsin versus males in its rural surroundings, respectively (244). Survival rate of 75% for breeding females over a 28- year study (1980–2007) in Wisconsin; no age-specific difference (minimum 3-yr-old in last year of detection, 4-yr-old, 5-yr-old, 6-7-yr-old, and 8-10-yr-old categories) in mortality rates between sexes in Wisconsin breeders (206). Survivorship in both sexes of breeding birds was unrelated to body mass or habitat in Wisconsin (244, 206), but experienced breeding females that dispersed to different nest sites in later years in Wisconsin may have lived longer than non-dispersing females (206). In contrast, females faithful to breeding sites had greater longevity in Florida (46). Maximum age of breeding birds in Wisconsin was 11 yr for both one male and one female, respectively (1; W. E. Stout, personal communication).

Roth et al. (151) reported 75% survival rate over a 110-days in winter for adult male and female Cooper’s Hawks in rural and urban Indiana (rates not separated by habitat).

Disease and Body Parasites

Diseases

West Nile Virus was isolated from a single specimen in Connecticut (247). In southeast Wisconsin, including city of Milwaukee, 88% of 42 breeding adults and 2.1% of 96 nestlings tested positive for West Nile Virus antibodies, but no adverse population effects detected (248).

The prevalence of flagellated protozoan, Trichomonas gallinae, (which causes respiratory disease ‘frounce', or trichomoniasis) in Cooper's Hawk nestlings is variable across different parts of the species' breeding range. Trichomoniasis infection rates of 85% and 9% were reported in nestlings in urban and non-urban areas of southeastern Arizona, respectively (110), and the disease caused 50% of deaths in urban nestlings there (5). Infection rates of T. gallinae in nestlings in urban and rural habitats in Wisconsin, North Dakota, and British Columbia were only 2.7%, and no deaths were linked to the infection (249), and only 2.5% of combined total (n = 157) of these nestlings with fall migratory birds of various ages in Minnesota, and nestling and breeding adults in North Dakota and Wisconsin were infected with T. gallinae (250).

Cooper’s Hawks admitted to rehabilitation center in Illinois regularly had (IgY) antibodies against the bacterium, Mycoplasma gallisepticum, a pathogen in feeder birds that causes eye disease conjunctivitis, and antibodies against the virus Avipoxvirus, which produces skin lesions. These disease agents are obtained by ingestion of prey and are more likely to occur in raptors such as Cooper’s Hawks that eat birds. Cooper’s Hawk antibody levels for M. gallisepticum were twice as high compared to 6 other raptors, but no signs of infection (251). Illinois researchers indicated that Cooper’s Hawks were able to mount adaptive response to these pathogens, and that pathological conditions due to infections are the exception in hawks.

Parasites

In Wisconsin, 94% of adults tested (n = 47) were infected with avian Hematozoa Leucocytozoon toddi (91%) and Haemoproteus sp. (62%). A single adult (male) was infected with microfilariae. In nestlings, infection rates were much lower, with only a 12% (n = 33) L. toddi infections (252). A high prevalence of L. toddi and Haemoproteus spp. infections was also recorded in 26 individuals trapped in northern New York, with higher infections in birds caught in late spring, compared with those caught in early spring. One individual had a Trypanasoma avium infection (253).

In Florida, two specimens were infected with trematodes including Strigea falconis, Neodiplostomum americanum, N. attenuatum, Ophiosoma microcephalum; nematodes including Capillaria sp., Cyathostomaamericana, Synhimantus hamatus, Tetrameres sp. Porrocaecum depressum, Cardiofilaria pavlovskyi, as well as with larval spirurids (254).

In Arizona, hematozoan infections were higher in adults than in nestlings, with no differences between the sexes. Infections did not appear to affect PVC (packed cell volume) or TS (total blood solid or the protein concentration in the plasma), both of which reflect a bird's health (255).

Choanal and cloacal aerobic bacterial flora include the following bacteria: coagulase-negative Staphylococcus/ Mircococcus spp. and Corynebacterium spp. (choanal isolates, in free-living birds), Corynebacterium spp. and Pasteurella spp. (choanal isolates, in captive birds), coagulase negative Staphylococcus / Micrococcus spp., Escherichia spp. and Salmonella spp. (cloacal isolates, in free-living hawks), Escheridia spp., coagulase-positive Staphylococcus and Streptococcus spp. (cloanal isolates, in captive birds).

Causes of Mortality

Nest predation, especially by raccoons (Procyon lotor; e.g., 224, 213, 185) and Great Horned Owl (Bubo virginianus; 84), probably most widespread and important cause of nestling mortality (raccoons will also take nesting adults), but largely unquantified. Nestling death or nest destruction by exposure (178), windstorm, or logging (212, 120), and starvation of youngest, smallest sibling (183, RNR, JB) also known or assumed. Disturbance by humans, unless prolonged (> 30 min), unlikely to bring desertion of eggs or young (256, RNR, JB).

In a 14-year study in Wisconsin, involving multiple and repeated sources of potential disturbance (e.g., attempts [often successful] to trap adults at all stages of nesting, climbs to nests to count egg and band young), including an estimated cumulative total of more than 3,000 visits over 3–4 months to 330 nests, only 4 (1.2%) nests were known to have failed due to researcher disturbance of extended visits (~1 hr). In all 4 instances, females, not males deserted; males tried unsuccessfully to incubate clutches for 7–10 d following mates’ desertion (256).

Male-male and female-female aggression in competition for and/or defense of breeding territories apparently common with occasional lethal results, at least in males (182, 46, 1, 206). In Florida, Millsap et al. (46) documented higher mortality in adult males outfitted with heavier radio transmitters (weighing 10 g) vs. males with lighter packages (6.5 g); researchers believed most of the males carrying heavier radios were killed by Cooper’s Hawks.

Researchers in Wisconsin observed a 1-yr-old female Cooper’s Hawk take a less than 1-wk-old nestling from a Wisconsin nest while the tending female was away from the nest (201).

Falconers take small numbers of nestlings (183).

One adult was found dead from a snakebite next to a den, occupied by an eastern cottonmouth (Agkistrodon piscivorus) and an eastern diamondback rattlesnake (Crotalus adamanteus) (257).

Range

Initial Dispersal from Natal Site

In Wisconsin, excluding Milwaukee, mean distance from natal site to breeding site 7.2 km (median 4.0 km, range 0.8–35.2) for males and 27.6 km (median 14.4 km, range 1.6–79) for females in Wisconsin (192, 1, 14). In Milwaukee, natal distances averaged 4.5 km (range 3.3–5.7) for males and 19.9 km for females (range 18.8–20.7) (185). Maximum natal dispersal distances in both sexes likely not habitat specific (including urban versus rural) in Wisconsin because statewide, saturated nesting habitats apparently invariant in habitat quality (14).

In Arizona (232), information collected from 34 radio-tagged fledglings showed that the young hawks were initially sedentary in the natal area, then moved relatively long distances from 11–13 weeks after hatching, and became sedentary again on winter home ranges. Females generally moved longer distances during the ‘exploring' period (average 6.8 km for females and 3.8 km for males), and on average, females settled further from their natal site (9.7 km) than their male counterparts (7.4 km; 258).

Greater natal dispersal distances in females than in males apparently results in reducing potential inbreeding as reports of such phenomena are rare; e.g., successful reproduction reported in a 3-year mating of grandson–grandmother in Wisconsin (192), and two instances of full sibling matings in Victoria, British Columbia (this study also reported two incidents of recruits breeding on their natal sites; 259).

Fidelity to Breeding Site and Overwintering Home Range

Birds frequently reoccupy nesting areas and breeding site fidelity often assumed in New York about 10% of (e.g., 39), but few data from marked birds. Banded pair occupied same site for 3 years in Wisconsin (192), at least 3 females at same site for 2 years each in Oregon (207). New birds often occupy an area formerly used by others (RNR, JB).

No information on winter home range fidelity but several marked birds of each sex captured or observed in Wisconsin in January within 0.8 km to 5 km of prior or subsequent year's breeding site (RNR), while 6 females banded in winter in Arizona later bred “nearby” (208).

In Wisconsin, breeding males show lifelong fidelity to nesting areas, while 23% of experienced females move to another nesting area in a different year. Range of dispersal distances for experienced females was 0.8–14.6 km (median 2 km, mean 3.7 km, n = 18) (260, 206, 14). Inter-year movements of among breeding territories occur in 2% of adult males in rural Florida (mean distance 0.6 km, 46), and in 3% of males exhibit breeding dispersal in Tucson, Arizona (258).

In Tucson, Arizona, about 10% of experienced females annually move among breeding territories (258). Much higher rate of 68% of females exhibited breeding dispersal in rural Florida (mean 4.2 km; 46).

Home Range

In breeding season, estimated at approximately 400–1,800 ha in New York, Michigan, Oregon, and New Mexico (166, 152). Home range was 784 ha for a radio-marked adult male in Wisconsin over a 3-month period (nestling through fledgling stages), with much variation by stage of breeding (193–571 ha), smaller 1-day ranges (mean 231 ha), and disproportionate use (88%) of small parts (12%) of home range (106).

In Tucson, Arizona, breeding male Cooper's hawks had home ranges of 13–131 ha (mean 65.5 ha, n = 9), with the size of an individual's home range generally decreasing with the number of years that the bird had occupied the territory. Home ranges observed in this study were smaller than in previous studies (784 ha for one male in Wisconsin; 106; 1,206 ha for males in New Mexico; 261). This is probably owing to the high abundance of prey near the nest, where the hawks preferred to hunt (138). Home ranges for Arizona Cooper’s Hawks during their first winter, calculated for 9 individuals, averaged 771 ha (232). In an apparent food limited landscape of rural Florida, breeding-season ranges for males was 1,460 ha in rural Florida (46).

In California, home range sizes of breeding males in urban landscapes were smaller—but not significantly so—in urban (481 ha) versus ‘natural’ habitats (609 ha); nonbreeding home range sizes averaged 238 ha (range 129–368) for males in urban habitat and 221 ha for one male in ‘natural’ landscape (262).

In southwestern Tennessee during winter, a single male had a home range of 331 ha and 4 females had an average range of 836 ha (126).

Population Status

Numbers

Although data from the North American Breeding Bird Survey (BBS) have limitations, especially for elusive species like the Cooper's Hawk, the population was estimated at 800,000 individuals for the United States and Canada from 2005 to 2014 (263).

On large areas that were carefully searched, including cities, 101–2,326 ha per active nest in western states (152, 67, 1), and 272–5,000 ha per active nest in midwestern and eastern states (181, 113, 13, 108), but numbers of nonbreeding birds on such areas unknown. Also, 800–1,000 ha per occupied nest area in non-riparian habitats in Arizona (208), and consistently ≤ 635–845 ha per active nest, 1985–1993, on one area of discontinuously wooded rural land in Wisconsin (113, JB). Distances between nests in California (183) suggest density near maxima of above range. In rural North Dakota, nesting densities were 1 nest/292 ha and 1 nest/395 ha (213), and in rural Florida, 1 nest/292 ha (46). Such rural metrics are similar to urban nesting densities (e.g., 1 nest/272 ha in Stevens Point, Wisconsin [113] and 1 nest/437 ha in Tucson, Arizona [10]). However, there is less variation in nesting densities among cities (varying about 4-fold between lowest and highest measures [notably, spacing between closest nesting pairs in cities about 200 m]) compared to variation of nesting densities among Cooper’s Hawk populations in rural habitat (varying by about 17 times; 137, RNR). Comparisons of nesting density among populations to reveal quality of habitat often do not consider confounding factors of time-specific environmental influence (e.g., poor weather, recovering population) or that smaller western birds might attain higher densities as body size in vertebrates is inversely related to density (2, 16, 137).

Trends

Considered “common” in West (152), where population was believed to be relatively stable (264, 265, 84), while historically varying in East mostly during the 1960s and 1970s (see below), but little information on comparative regional numbers during that period. Roadside surveys, Christmas Birds Counts, and similar counts hampered by problems of detection (hence sample size) and identification (266, 267, 181). Decline in eastern breeding population suspected in early 1900s (24), and autumn migration counts showed downward trend after mid-1930s (268, 269, significantly so about 1947–1950 (264, 270). Concurrent declines in annual reproductive success (see above) noted by Henny and Wight (128). For correlates of decline, see Conservation and Management: Effects of Human Activity.

BBS data for this species during 1966–2015 (271), in states with robust sample sizes, show the following estimated trends (mean percent annual change): 3.3% in Georgia, 1.4% in Michigan, 3.9% in Ohio, 4.4% in Pennsylvania, 3.6% in Wisconsin, and 1.2% in California. These BBS data, along with population trajectory data (since late 1980s) from North Dakota (272), Tucson, Arizona (112), Albuquerque, New Mexico (74), and Chicago, Illinois (273), suggest broad and significant increases in Cooper's Hawk populations in most regions over the past 50 years. Similarly, trend analyses from Breeding Bird Survey for 1966–2015 indicated a survey-wide increase of 2.2% per year (271). Christmas Bird Count data suggest a similar broad upward trend in the United States (Figure 11).

Both abundance (as assessed by migration counts [270]) and reproductive success (241) rose in mid- to late 1960s; current reproductive output equals pre-1945 levels in many areas, including Arizona (258, 112), California (1972–1975, B. J. Walton, personal communication, 262), Florida (46), Missouri (121), New Mexico (74), North Dakota (213, 186), and Wisconsin (120, 223, 113, 15). In longest study spanning almost 40 years, Wisconsin population likely growing during 1980s on basis of significant increase in migration counts at Great Lakes and Wisconsin watch sites (274, 275), high indices of annual productivity statewide, and high rates of annual survival of breeding males during that decade (250, 205). Migration counts slowed and/or had stabilized from 1990s through 2010 (276), while at same time population counts of breeding pairs had stabilized on a southeastern Wisconsin study plot, excluding Milwaukee (205, RNR). After breeding population likely saturated most Wisconsin habitats, Cooper’s Hawks then colonized metropolitan Milwaukee in early to mid-1990s, and subsequently increased at a relatively rapid rate during late 1990s to mid-2000s; nesting densities increased 25-fold (with concomitant increase in brood size/nest) from time of colonization to likely stable breeding population that saturated Milwaukee nesting habitats by about 2015 (108, W. E. Stout, RNR). Migration counts from coastal and inland flyways in the eastern states suggest significant recent increases in this region (Table 1).

Nenneman et al. (213) and Driscoll and Rosenfield (187) reported stable nesting populations of Cooper’s Hawks in rural, central North Dakota in 1990s, and in urban habitat of Grand Forks during 2004–2014 of that state, respectively.

Population Regulation

Not well studied. Human impacts (shooting, pesticides) were likely important during early and mid-1900s (see Conservation and Management: Effects of Human Activity). Collisions with windows may be frequent, but magnitude of their impact is not known; 8 (6%) of 136 mortalities recorded in BBL files up to 1979 apparently due to vehicular collisions (245). Need more information on natural factors in both urban and natural areas; starvation and predation by Great Horned Owl and Red-tailed Hawk (Buteo jamaicensis) are probably important in some locales (137). Impacts of disease factors via common predation of feeder birds throughout range needs investigation (1).

Recommended Citation

Rosenfield, R. N., K. K. Madden, J. Bielefeldt, and O. E. Curtis (2019). Cooper's Hawk (Accipiter cooperii), version 3.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.coohaw.03