Unfounded Assumptions about Diet of the Cooper's Hawk

Abstract

Prior analyses and reviews of diet in the Cooper's Hawk (Accipiter cooperii) have suggested that avian prey usually provides a majority of items and biomass, but this interpretation may be compromised by methodological problems. In Wisconsin, we found that indirect collection of prey remains near nests (92% birds) overestimated the proportion of avian items in comparison to direct observation of prey deliveries to nestlings (51-68% birds). Previous studies using indirect methods may have overstated the frequency of birds in the diet. Also, most avian items brought to nestlings in Wisconsin, as elsewhere, were young birds. Some prior studies relying on indirect methods and using prey species' adult mass to calculate avian biomass have probably accentuated the methodological bias toward birds among prey remains. In Wisconsin, birds provided a minority of prey biomass (4046%) in two of our three sampling pools. Proportions of avian items and biomass are highly variable in the small set of other direct dietary studies of the Cooper's Hawk, all from the breeding season. Because of methodological, seasonal, geographic, and other limitations, existing data do not warrant an assumption that birds prevail in the diet. Because vulnerable ground foraging animals (as well as young birds) appear to constitute most of the breeding season diet, we also question the assumption-based mainly on breeding season studiesthat Cooper's Hawks feed principally on prey of high agility, as proposed in some theories of reversed sexual size dimorphism.