Influence of Sibling Competition on Nestling Sex Ratios of Sexually Dimorphic Birds

Abstract

Theoretically, the ratio of male to female offspring should equal the inverse ratio of the cost of their rearing. For birds exhibiting brood reduction, the cost of raising offspring sexually dimorphic in size may not be independent of the sex composition of the brood if males and females are not equally affected by sibling competition or are not equally effective at competing with a sibling of the opposite sex. Parents may thus manipulate the sex ratio of their brood to achieve an optimal combination of the sexes and to maximize their fitness. By examining the effects of sex, growth, and hatching asynchrony on the relative size of sibling bald eagles, I show that the probability of brood reduction is not equal among broods of different sex composition. The hatching sequence of male first and female second was predicted to have the greatest chance of experiencing nestling mortality. This type of brood is rare in bald eagles because there is a sex-dependent hatching sequence whereby the first egg in mixed-sex clutches is predominantly female (93%). In contrast, golden eagle broods of female first and male second appear to be the combination most likely to result in brood reduction. Golden eagles do not adjust their prenatal sex ratio, but there is a postnatal sex bias in favor of females caused by brood reduction in years of poor food availability.