Sensitivity to Disturbance
Species once known as the ‘chicken hawk', a far worse ‘villain' than its smaller ‘blood-thirsty' cousin, the Sharp-shinned Hawk (Accipiter striatus; 24). Thus, heavily persecuted during this time and became shy, elusive and sensitive to disturbance at the nest. Presently, persecution is negligible, and the species is far more tolerant of human activity at the nest (see also direct human / research impacts). In fact, this species is now the most common diurnal raptor in many urban areas.
Shooting and Trapping
Effects believed historically significant in the east, where mortality rates from shooting for first-year birds were estimated at 28–47% in 1929–1940, and 12–25% in 1941–1957 (128). Pole trapping and shooting at game farms in New York (in late 1940s) took mostly immature females (39). Little legal protection until the 1960s. Shooting is now considered unimportant (277, 270), but may still occur at occasional nests in the eastern U.S. (RNR, JB), western U.S. (e.g., 65, K. Madden, personal communication), and for western populations overwintering in Mexico (152, 79).
Pesticides and Other Contaminants
Concurrent declines in reproductive success (128) and migration counts (270) in the late 1940s and 1950s attributed to organochlorine contaminants, especially DDT and its metabolite DDE. Decreases of 7–19% in eggshell thickness (278, 279, 280) were significantly correlated with elevated DDE levels in eggs, egg breakage, and egg desertion in Arizona and New Mexico; even higher DDE levels occurred at the same time in Pennsylvania and New York (264). Organochlorine levels in adults, occasionally fatal (281, 282), may have prompted aberrant parental behavior (279). After regulation of DDT in the early 1970s, residues below levels believed to cause shell thinning were reported for eggs from 5 eastern states in 1980 (283), including Wisconsin, where little or no thinning occurred and hatching success was high (120).
Contaminants other than DDE (e.g., dieldrin, PCBs, mercury and other heavy metals) were also present in eggs, but with unknown effects (264, 120, 283). A few recent cases of organophosphate poisoning reported (99) but effects on populations unclear, as are consequences of pesticide use in Mexico for western birds wintering there (152). Probably negligible.
Detectable levels of chlorinated hydrocarbons found in all analyzed eggs (n = 18 eggs) collected in Ontario between 1986 and 1989. Although mean eggshell thickness was significantly less (–8%) than pre-DDT, reductions were not considered a factor in reproductive success (227).
Some raptors that feed on killed or injured game may be at risk of accumulating high levels of lead in their blood. One study of Cooper's Hawks caught as outbound and return migrants in New Mexico found traceable, although most likely sub-clinical, amounts of lead-contamination (284).
Blood plasma samples from live Cooper’s hawks within the urban areas of Vancouver, Canada contained PCBs, organochlorine pesticides dieldrin, trans-nonachlor, DDE, and flame retardant polybrominated diphenyl ethers (285). In this study, concentrations of PCBs increased with the level of urban development and evidence suggests that contaminants may have some effects on the health of individual Cooper’s Hawks.
Collisions with Stationary/Moving Structures or Objects
Collisions with human-made objects is the cause of 70% of deaths in urban Cooper's Hawks (5). Out of 115 specimens examined by Roth et al. (286), 26 (23%) had naturally-healed pectoral-bone fractures, presumably from impact injuries, suggesting that some hawks may survive such injuries in the wild.
Degradation of Habitat
Logging may alter suitability of nesting or foraging habitats as well as prey populations on local or regional scales, but magnitude and seasonality of such impacts remain uncertain (152). Breeding and nest site habitats are diverse, and apparently not limiting in many areas (99), including a suburban Wisconsin site (RNR, JB; J. Affeldt, personal communication) where density equals the rural maximum of 331 ha per active nest in southeastern Wisconsin conifer plantations (99). In Arizona, reproductive success was lower in heavily grazed than in lightly grazed riparian habitats; correlation with avian prey populations suggested (65).
Species used by some falconers but taking of wild birds has had little impact on wild populations. In a 1992 survey, an estimated 202 individuals were used for falconry in North America, about 70% of these taken as nestlings and 30% trapped as passage birds (287). Known as a temperamental species, difficult to train effectively; sturdier than the Sharp-shinned Hawk and “even more tyrannical, hysterical, and demanding than the Goshawk” (288). McElroy (289) and McLean (290) provide details of training and prey.
Direct Human/Research Impacts
Recoveries of banded Cooper's Hawks exposed to frequent handling and to research personnel were significantly more likely than recoveries of nestlings not disturbed as frequently (291), suggesting habituation to humans may jeopardize survival in this species. In Florida, Millsap et al (46) saw an unequal distribution of mortality in males equipped with (11g) radio transmitters, with most deaths occurring in the pre-breeding period. After 1998, when males were equipped with 6 g transmitters, there was no seasonal bias in mortality.
Intensive studies of breeding Cooper's Hawks (Wisconsin 1980–1993 and New Mexico 2010–2019) have involved multiple and often repeated sources of potential researcher disturbance over at all stages of nesting: nest-finding visits and trapping of both sexes using lure birds at the pre-laying and nestling stages; multiple visits, clutch counts, and attempts to trap adults using an owl as decoy near nests during incubation; additional visits, climbs to count and band nestlings (Wisconsin), and trapping of fledglings using lure animals in bal chatri traps. These procedures were repeated in subsequent years at many of the same nesting areas (which often held the same adults), on study sites often near areas of frequent use by other humans. Although not every nest was subjected to the full array of these activities, Rosenfield and Bielefeldt estimated a cumulative total of > 3,000 visits to 330 nests in Wisconsin, of which only 4 (1.2%) are known to have failed because of researcher disturbance. Trapping during pre-incubation has not resulted in desertions or other discernible impacts (140). Research activities have not contributed to mortality of any birds or nest failure at any of the New Mexico nests (Millsap and Madden, personal communication).