Prey sizes of male and female northern goshawks

A common explanation proffered for reversed sexual size dimorphism among raptors is that size dimorphism allows the partitioning of prey size between the sexes and reduces intersexual competition for food. During the breeding seasons of 1990, 1991, and 1992, we compared prey captured by male and female northern goshawks (Accipiter gentilis) at 16 nests in northern Arizona. On basis of 192 prey items captured by males and 46 captured by females, we found no difference between the sexes in mean mass of prey captured, or in the distribution of prey sizes. There was no difference between the sexes in capture rates of mammals or birds; mammals accounted for 85% and 79% of the prey captured by females and males, respectively. The sexes had high dietary overlap (92%, Pianka's index), but males used the available prey species less equitably than did females (male = 0.37, female = 0.51; min 0.0, max 1.0). Prey captured in different foraging zones did not differ between the sexes. Our findings add to a growing collection of data investigating prey partitioning as an explanation for reversed sexual size dimorphism.