Pair and group reproductive success, polyandry, and co-operative breeding in Harris' hawks

Abstract

Average size of Harris' hawk Parabuteo unicinctus social units in New Mexico was 2.7 individuals, and pairs were most common (49%). Groups of >2 hawks included both adult-(74%) and immature-plumaged (26%) members. Immatures rarely provided food to nestlings, but adult supernumeraries did. At least in SE New Mexico, Harris' hawk groups consist primarily of a monogamous pair with "helpers'. Pairs and groups showed no differences in clutch size, number of young produced per successful nest, or number of offspring fledged per year. Pair nests failed less often (16%) than group nests (46%) during the incubation period. Groups reared larger offspring and tended to initiate 2nd nests more frequently than pairs. The overall lack of correlation between reproductive output and group size suggests that kin selection has not been a major influence in the evolution of the Harris' hawk breeding system. -from Author