Skewed Brood Sex Ratio and Sex-Biased Hatching Sequence in Harris's Hawks

Abstract

Nestling sex ratios in Harris's hawks show a significant bias toward males at the age of sexing, as predicted by Fisher's equilibrium hypothesis. Our data, however, do not support current hypotheses of parental manipulation of offspring sex ratios in response to varying environmental or social cues. The overall bias in offspring sex ratios was due to a highly significant skew toward males in first-hatched nestlings. We propose that sex-dependent hatching sequence may represent a proximal mechanism that upholds an adaptive sex-ratio skew toward males, the less expensive sex. If this mechanism were not employed by Harris's hawks, we submit that the dimorphic-size advantage of female nestlings added to the developmental advantage of hatching first might result in maladaptive brood reduction. Our observations show that nests in which males hatch first fledged significantly more young than nests in which females hatch first. Therefore, we suggest that female breeders may enhance their fitness by skewing the sex ratio of the first-hatched offspring toward males