Nestling sex-ratio variation in western bluebirds

We tested five hypotheses for facultative manipulation of sex ratios in a population of Western Bluebirds (Sialia mexicana) studied for 13 years in California. Based on 2,187 nestlings from 549 broods, the overall mean (+/- SE) proportion of males was 51.9 +/- 1.1%, not significantly different from 50%. Sex ratios of nestlings were significantly biased in only 1 of 13 years, and we failed to detect any significant bias related to brood reduction, breeder female age, presence of helper males, first-egg date, condition of the breeder female, or annual differences in environmental conditions. These results allow us to reject all five hypotheses, including: (1) nestling size dimorphism and local resource competition, both of which predict a female-biased sex ratio; (2) local resource enhancement via the ''repayment model'' of Emlen et al. (1986), which predicts a more male-biased sex ratio than we observed; and (3) seasonal changes in either maternal condition or nestling condition, both of which predict shifts in sex ratios not observed in our data. These results prompt us to urge caution when reporting sex-ratio biases in natural populations and when interpreting published studies, many of which suffer from small sample sizes, post hoc analyses, and insufficiently conservative statistical tests. Sex-ratio biases in birds are uncommon, and considerable data will be necessary to determine which hypotheses (if any) are sufficiently robust to regularly select for avian sex ratios that deviate significantly from 50:50.