Offspring sex ratios correlate with pair-male condition in a cooperatively breeding fairy-wren

We examined sex allocation patterns in island and mainland populations of cooperatively breeding white-winged fairy-wrens. The marked differences in social structure between island and mainland populations, in addition to dramatic plumage variation among males both within and between populations, provided a unique situation in which we could investigate different predictions from sex allocation theory in a single species. First, we test the repayment (local resource enhancement) hypothesis by asking whether females biased offspring sex ratios in relation to the assistance they derived from helpers. Second, we test the male quality (attractiveness) hypothesis, which suggests that females mated to attractive high-quality males should bias offspring sex ratios in favor of males. Finally, we test the idea that females in good condition should bias offspring sex ratios toward males because they are able to allocate more resources to offspring, whereas females in poor condition should have increased benefits from producing more female offspring (Trivers-Willard hypothesis). We used molecular sexing techniques to assess total offspring sex ratios of 86 breeding pairs over 2 years. Both offspring and first brood sex ratios were correlated with the pair-male's body condition such that females increased the proportion of males in their brood in relation to the body condition (mass corrected for body size) of their social partner. This relation was both significant and remarkably similar in both years of our study and in both island and mainland populations. Although confidence of paternity can be low in this and other fairy-wren species, we show how this finding might be consistent with the male quality (attractiveness) hypothesis with respect to male condition. There was no support for the repayment hypothesis; the presence of helpers had no effect on offspring sex ratios. There was weak support for both the male quality (attractiveness) hypothesis with respect to plumage color and the maternal condition hypothesis, but their influence on offspring sex ratios was negligible after controlling for the effects of pair-male condition.