A Study of the Ecological Bases of Cooperative Breeding in the Harris' Hawk

We compared ecological variables of pairs and larger groups of Harris' Hawks (Parabuteo unicinctus) in southeastern New Mexico in an effort to test several ecologically based hypotheses proposed to explain cooperative breeding. Univariate and multivariate analyses of 21 habitat variables revealed no differences (P > .1) between the nest areas of pairs and groups. Habitats not occupied by hawks had significantly (P < .05) fewer and smaller mesquite trees and greater litter cover than occupied areas. Densities of lagomorph fecal pellets (an index of abundance of the primary prey species) was slightly greater (P > .1) in group areas than pair areas in summer, but the pattern was reversed in winter. Pair ranges had significantly (P < .05) fewer woodrat (Neotoma, an alternate prey) nests than group areas in all seasons. Rabbit femurs from prey remains of groups were slightly, but not significantly, longer than those from pair remains. Although general hypotheses cannot be fully evaluated in studies of single species, the data we obtained do not provide clear support for any of several current hypotheses concerning cooperative breeding in the Harris' Hawk. Instead, group living in this hawk appears to be related to dependence on large, elusive prey in a manner analogous to the social system of wolves (Canis lupus) and wild hunting dogs (Lycaon pictus). We conclude that this species' social system is based on (1) extended parental care, where young birds remain with older group members during their first year or years, and (2) cooperation, where group hunting and prey—sharing provide a critical advantage to each individual.