Nestling sex ratio variation in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus)

We examined sex ratio variation in the cooperatively breeding acorn woodpec ker (Melanerpes formicivorus) over a 27-year period in central coastal Cali fornia. The sex ratio of eggs is even or possibly biased toward females. Ho wever, hatching asynchrony and brood reduction differentially favor the sli ghtly larger males such that the sex ratio when young become self-sufficien t is 54.0% males, slightly but significantly different from 50:50, This bia s, or at least the differential mortality that produces it, does not appear to be explainable by any of the major hypotheses for an overall sex bias. These include the repayment model, local resource competition, and sexual s ize dimorphism. all of which predict a female bias or a male bias significa ntly smaller than that observed. On an individual level, we found no eviden ce for facultative sex ratio manipulation related to annual differences in the acorn crop, apparent seasonal declines in food availability, or differe nces in territory quality. We also found no evidence for a non-binomial dis tribution of sexes within broods despite the potential fitness benefits of creating large coalitions of same-sexed siblings. Males inherit their natal territories more frequently than do females, and we speculate that birds m ay invest more in sons either to increase the probability that they will be able to pass along their territory to their descendants or to facilitate t heir sons' ability to compete as future cobreeders. Alternatively, acorn wo odpeckers may be unable to fully compensate for the ancillary bias caused b y differential juvenile survivorship, leading to unequal investment in the sexes contrary to the prediction of Fisher. Opposite to what has been found in other birds and mammals, the faster growth and larger size of male nest lings appears to confer a survival benefit rather than a cost.