KINSHIP, PROMISCUITY, AND COMMUNAL BREEDING IN THE ACORN WOODPECKER

1. Acorn woodpeckers typically live in permanent social groups in which most adults help to incubate and feed the young of a single nest. The selective basis of communal breeding in this species is examined. 2. At the study area in New Mexico, most juveniles did not remain on their natal territories during subsequent breeding seasons. There was also a high rate of turnover among adults within each group. The groups were not composed entirely of extended families, and unrelated adults fed young. Kin selection alone does not appear to be adequate to explain all instances of communal breeding. 3. Immigrants that joined groups before courtship in the spring participated in all phases of reproduction in their new groups. However, immigrants that joined groups after eggs were laid did not incubate or feed the young. This, combined with the high rate of turnover within groups, indicates that hypotheses based upon individual benefits through increased production of young and/or reciprocity are also not sufficient for this species. 4. An examination of the reproductive behavior within groups suggests that the mating system may be promiscuous. It was not possible to identify which individuals were the parents of the young, and all group members normally participated in every phase of reproduction. A promiscuous mating system means that each group member could make a genetic contribution to the young and that all would help to raise the young to protect their genetic investment. 5. Larger groups successfully fledged more young than did smaller groups, and promiscuity may be an individual strategy to insure that all individuals remain within the group and participate in reproduction. © 1979 Springer-Verlag.