Intragroup dynamics of a cooperative breeder: An anlysis of reproductive roles in the acorn woodpecker

Summary The acorn woodpecker is a cooperative breeder generally thought to breed promiscuously within groups, but alternative patterns of reproductive investment can be expected and remain to be examined. With this feature of mating system in mind, we studied the reproductive roles of individuals in a single group of this species in central coastal California over a three-year period. Particularly detailed observations were made in 1979, when the group consisted of two potentially breeding sibling males and a single breeding female. One male (298) contributed significantly more than 297 in virtually all aspects of nest and fledgling care. These include (1) feeding nestlings, (2) nest sanitation, (3) nocturnal incubation and brooding, (4) feeding fledglings, and (5) escorting fledglings. In the few observed and rarely occurring dominance interactions, 298 prevailed. These results suggest that 298 had a higher probability of fathering the group's two 1979 offspring than did his brother. Thus, the two males differed substantially in their parental roles within the group, and very likely in reproductive roles, also. At present stage of knowledge, monogamy in groups with one female and more than one potential reproductive male is no less likely than promiscuity. These findings are discussed with respect to the evolution of cooperative breeding in this species.