Manipulation of offspring sex ratio by second-mated female house wrens.

In 1973, Trivers and Willard proposed that offspring sex ratio should be associated with the quality of parental care likely to be provided to the offspring. We tested this hypothesis by comparing fledgling sex ratios in nests of first- and second-mated female house wrens (Troglodytes aedon). In our Wyoming population, second-mated females typically receive little or no male parental assistance and fledge fewer and lower-quality young compared with first-mated females. Assuming that being of lower quality has stronger negative effects on the future reproductive success of males than that of females in this polygynous population, we predicted that fledgling sex ratios in the nests of second-mated females would be female-biased compared with the fledgling sex ratios of first-mated females. Additionally, we asked whether any sex bias at fledging could have resulted from male-biased nestling mortality caused by sex-biased parental provisioning. As predicted, mean fledgling sex ratios in nests of second-mated females were more female-biased than fledgling sex ratios in nests of first-mated females. However, we found no evidence of either sex-biased nestling mortality or sex-biased parental provisioning. These findings suggest that females are responding to their status as second-mated females and to the associated low-quality parental care that their young are likely to receive by producing female-biased clutches rather than manipulating the offspring sex ratio through sex-biased nestling mortality.