Paternal song complexity predicts offspring sex ratios close to fledging, but not hatching, in Song Sparrows

Sex allocation theory predicts that population sex ratios should be generally stable and close to unity, but individuals may benefit by adjusting the sex ratio of their offspring. For example, females paired with attractive males may benefit by overproducing sons relative to daughters, as sons inherit their fathers' attractive ornaments ("sexy son" hypothesis). Similarly, if compatible gene effects on fitness are more pronounced in males than females, genetically dissimilar mated pairs may enhance fitness by overproducing sons ("outbred son" hypothesis). We tested these hypotheses in Song Sparrows (Melospiza melodia) by examining offspring sex ratios of 64 complete families shortly after hatching ("early-stage") and again shortly before fledging ("late-stage") in relation to paternal song complexity and the genetic similarity of social mates. Neither early nor late-stage offspring sex ratio was related to parental genetic similarity. Nests of males with larger song repertoires contained more male-biased broods by the late-stage nestling period, but not in the early-stage nestling period. These findings suggest that attractive males may be better able to successfully raise male-biased broods, but not that females adaptively adjust primary sex ratios in response to their social mate's attractiveness. © 2010 by the Wilson Ornithological Society