Sex allocation theory predicts that females should bias the sex ratio of their offspring in response to differences in the reproductive value of sons and daughters. Offspring reproductive value may vary as a result of mate attractiveness, mate condition, or habitat quality; therefore, females should bias sex ratio in response to these attributes. Male plumage colouration, for example, may signal the direct (e.g., parental care) and indirect (e.g., good genes) benefits a female can gain by mating with a particular male. If males inherit good genes and receive greater parental care from a high quality dad, sons will have a higher chance of reproductive success. Hence, a female mated to a high quality male may produce a male-biased brood. Male mountain bluebirds (Sialia currucoides) display full body UV-blue structural plumage colouration, which is associated with male attractiveness and condition. In this study, I investigated whether female mountain bluebirds produce sex-biased broods in response to mate attractiveness or environmental conditions. Brood sex ratios were related to male plumage colouration, with male-biased broods resulting when females mated with brightly coloured males. This relationship was seen only in broods produced by older females, suggesting that a female’s ability to assess male plumage and bias offspring sex in response may be related to her breeding experience. Brood sex ratios also became male-biased with increasing elevation, and male-biased broods were provisioned more frequently than female-biased broods when the parents were older birds. In conclusion, these results support the hypotheses that female mountain bluebirds bias offspring sex in response to both mate attractiveness and environmental conditions.
