No evidence of sex ratio manipulation by black-throated blue warblers in response to food availability

Sex allocation theory predicts that females should bias their offspring sex ratios when the fitness benefits of producing sons or daughters differ depending on rearing environment. The Trivers-Willard hypothesis proposes that whether females produce more sons or daughters depends on food availability via both intrinsic maternal condition and differing reproductive potential (typically from mating system structure) for sons versus daughters. However, tests of its key predictions are often based on untested, implicit assumptions that are difficult to quantify, especially in migratory animals. In a 5-year study, we manipulated food availability in low- and high-elevation forest to test the Trivers-Willard hypothesis in the migratory black-throated blue warbler (Setophaga caerulescens). We found that the population-wide offspring sex ratio was significantly male-biased (population mean: 0.58), which was driven by an overproduction of sons in high-elevation forest (high-quality habitat mean: 0.59). Yet, we found no effect of food availability on offspring sex ratio from either natural variation or supplemental feeding. Sex-specific developmental costs did not differ for sons and daughters reared under low and high food availability. These results suggest that female black-throated blue warblers do not manipulate offspring sex ratios in response to food availability and are not consistent with the predictions of the Trivers-Willard hypothesis. This study highlights challenges of examining mechanisms driving patterns in offspring sex allocation in migratory species for which both the costs of rearing and relative fitness benefits of sons and daughters cannot be tracked into adulthood. Birds can optimize their fitness return on parental investment by biasing offspring sex ratios. When the costs and benefits of raising sons or daughters differ under low and high food availability, females could produce either more sons or daughters depending on those conditions. Black-throated blue warbler offspring sex ratios were male biased in high quality habitat. However, we found no evidence that food-supplemented females or females on territories with higher caterpillar abundance produced more sons than daughters as predicted by the Trivers-Willard hypothesis. The relative costs of producing sons or daughters did not differ under low and high food availability, but we could not directly examine differences in relative benefits. Findings indicate that this migratory bird does not adjust sex ratios in response to food availability and highlights the need for evaluating future fitness benefits of sons and daughters in migratory species.