Sex allocation and nestling survival in a dimorphic raptor: Does size matter?

Fisher's theory predicts equal sex ratios at the end of parental care if the costs and benefits associated with raising each sex of offspring are equal. In raptors, which display various degrees of reversed sexual size dimorphism (RSD; females the larger sex), sex ratios biased in favor of smaller males are only infrequently reported. This suggests that offspring of each sex may confer different fitness advantages to parents. We examined the relative returns associated with raising each sex of offspring of the brown falcon Falco berigora, a medium-sized falcon exhibiting RSD (males approximately 75% of female body mass) and subsequent sex ratios. Female nestlings hatched either first or second did not receive more food nor did they hatch from larger eggs or remain dependent on parents for longer periods than male offspring from these hatch orders. Together with previous studies this result indicates that even in markedly dimorphic species, the required investment to raise the larger sex is likely to be less than that predicted by body size differences alone. Moreover, among last-hatched nestlings, both sexes faced a reduced food allocation and suffered a slower growth rate and thus final body size, with a concurrent increased probability of mortality. For last-hatched females the reduction in food allocation was more marked, with complete mortality of all last-hatched female nestlings monitored in this study. Once independent, males of any size but only larger females are likely to be recruited into the breeding population. The sex-biased food allocation among last-hatched offspring favoring males thus reflects the relative returns to parents in raising a small member of each sex.