Models of sex allocation theory suggest that when sons and daughters have different reproductive values to their parents, parents overproduce offspring of higher value to maximize their reproductive success. Empirical testing of these models remains inconclusive. We investigated whether parents of the Azure-winged Magpie (Cyanopica cyana), a cooperatively breeding bird, controlled offspring sex ratios and whether they exhibited sex-specific food allocation among nestlings. Our results showed that offspring sex ratios at hatching and fledging did not deviate from equivalent ratio in both cooperative groups and bi-parental nests. No factors were significantly correlated with offspring sex ratios at hatching, indicating that parents did not control their offspring sex ratios. However, parents adopted two postnatal strategies to control the growth pattern of nestlings. First, they delivered more food to their sons than their daughters to ensure the survivorship of their sons. Second, they adjusted the feeding rates and food allocation time per feeding bout according to the offspring sex ratios and positively responded to the begging behavior of nestlings to increase the survivorship of the daughters. Our findings suggest that the postnatal strategies adopted by the parents, in terms of sex-specific allocation of food to offspring, may play a greater role than the prenatal strategies, i.e., the control of offspring sex ratios, in the enhancement of reproductive success.
