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

Abstract

We examined sex ratio variation in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus) over a 27-year period in central coastal California. The sex ratio of eggs is even or possibly biased toward females. However, hatching asynchrony and brood reduction differentially favor the slightly larger males such that the sex ratio when young become self-sufficient is 54.0% males, slightly but significantly different from 50:50. This bias, 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 size dimorphism, all of which predict a female bias or a male bias significantly smaller than that observed. On an individual level, we found no evidence for facultative sex ratio manipulation related to annual differences in the acorn crop, apparent seasonal declines in food availability, or differences in territory quality. We also found no evidence for a non-binomial distribution 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 may 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 their sons' ability to compete as future cobreeders. Alternatively, acorn woodpeckers may be unable to fully compensate for the ancillary bias caused by 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 nestlings appears to confer a survival benefit rather than a cost.