A Comparative Study of the Breeding Ecology of Harris' and Swainson's Hawks in Southeastern New Mexico

I compared productivity, habitat, and diet of breeding Harris' (Parabuteo unicinctus) and Swainson's hawks (Buteo swainsoni) to examine predictions that cooperatively breeding birds should produce fewer young and be more specialized in their use of habitat or food than closely related noncooperatively breeding species. Reproductive success of Harris' Hawk (1.73 young/year) and Swainson's Hawk (1.67) breeding units was similar. Harris' Hawks tended to exploit habitats with greater mean densities of large mesquites, less mean grass cover, and more exposed ground than Swainson's Hawks; but both species used a broad range of the habitats available. Swainson's Hawks were opportunistic generalists taking the most available prey, while Harris' Hawks were relative specialists on larger mammals and some birds.My data do not support habitat saturation or resource localization as important to the evolution of cooperative breeding in the Harris' Hawk.