Female-Biased Delayed Dispersal and Helping in American Crows

ABsTRACr.-Observation of a resident population of Western American Crows (Corvus brachyrhynchos hesperis) over five years revealed a social structure that deviates from that of a majority of cooperative breeders, including other corvids. Breeding density was unusually high (0.8 pairs/ha). Core areas occupied by pairs and families were small, overlapped extensively with those of neighbors, and were not defended against conspecifics. A nonbreeding flock was resident on the study site. Juvenile dispersal patterns were highly variable; individuals dispersed at various ages after two months postfledging. They either joined the nonbreeding flock or left the study area. Some individuals delayed dispersal for one or more years, while some returned home after extended absences. As yearlings, more females than males were resident in their natal core area during the breeding season. Most, but not all of those at home served as helpers. More females than males assisted their parents in breeding. The female bias in dispersal and helping is unusual. It suggests that the costs and benefits associated with these behaviors differ between the sexes, and may be different from those postulated for many other cooperative breeders. Received 28 January 1991, accepted 13 January 1992.