Female mountain bluebirds (Sialia currucoides) paired to more colourful males produce male-biased broods

Sex allocation theory predicts that females should bias the sex ratio of their offspring in response to differences in the reproductive value of sons versus daughters. Consistent with this prediction, females of many species appear to bias offspring sex ratios in response to mate attractiveness and condition. Male mountain bluebirds (Sialia currucoides) display full body UV-blue structural plumage colouration, which is associated with attractiveness, condition, and reproductive success. Over four breeding seasons, we found females paired with more colourful males produced increasingly male-biased broods and provisioned offspring at a higher rate. Surprisingly, however, we also found females with duller plumage and those mated to first-year males produced more male biased broods. These results provide support for sex allocation in mountain bluebirds and suggest female reproductive decisions may be influenced by the attractiveness of her mate. However, this system is clearly complex and more work is needed to understand the roles of male age and female colouration in the signalling systems of mountain bluebirds.