Combined influence of maternal and paternal quality on sex allocation in red-capped robins

Sex allocation theory predicts females will adaptively manipulate sex ratios to maximize their progeny's reproductive value. Recently, the generality of biased sex allocation in birds has been questioned by meta-analytic reviews, which demonstrate that many previously reported significant results may simply reflect sampling error. Here, we utilize a robust sample size and powerful statistical approach to determine whether parental quality is correlated with biased sex allocation in red-capped robins. Indices of maternal quality (including interactive effects of age and condition) were strongly related to sex allocation. These relationships were in the predicted directions, with larger effect sizes than those of previous studies in this field. There were also paternal correlates, involving age and the source of paternity. We propose that biased sex allocation occurs in this species, and is maintained by differing production costs of each sex and genetic benefits to females of producing sons when fertilized by high-quality males.