Reproductive Success, Group Size, and the Evolution of Cooperative Breeding in the Acorn Woodpecker

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Abstract
Acorn woodpeckers are cooperative breeders which live in family groups of up to 12 adults. Analysis of the reproductive success and subsequent survivorship of young within groups indicates that (1) yearly variation in conditions, (2) prior group history, (3) territory quality and (4) group size all contribute substantially to the variability of these data. Of these, (1) and (2) are the most important. An analysis of covariance of reproductive success vs. variables (1)-(4) above indicates that when the effects of the other variables are controlled, only yearly variation in conditions and prior group history are consistently significant in their effects on reproductive success. Reproductive success per group, controlling for variables (1)-(3), increases with group size up to size 7-8 but reproductive success per bird declines with increasing group size. Groups of size 7-8 produce the most total offspring, but simple pairs produce more young per individual than groups of any larger size. Two females nesting together produce more total offspring but fewer offspring per female than those nesting by themselves. A review of previously published data indicates that pairs are either the most productive group size or as productive as any larger group size in a wide variety of other cooperative species. The few data available further suggest that adult survivorship does not increase significantly with increasing group size in cooperative species. For species in which this is the case, group living cannot have evolved as a response to ecological factors favoring group exploitation of resources or group defense against predators. Instead, individuals must be ecologically forced to accept the option of group living in order to maintain access to a localized and limited resource without which reproduction and survivorship are severely impaired. For most cooperative species it is not known what this resource might be, but in acorn woodpeckers the localized resource may be storage trees in which groups store acorns in the fall.