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Abstract

Maternally derived androgen hormones concentrate in avian egg yolks as the yolks grow on the female's ovary, possibly forming a basis for important maternal effects in birds. In the American kestrel (Falco sparverius), experimental elevation of yolk androgens in the first-laid egg of a clutch (a-egg) to the concentrations found naturally in a clutch's later-laid eggs reduces the growth rate of a-egg nestlings compared to controls. These findings, together with discoveries from other species that the effects of yolk androgens on growth of female nestlings may differ from their effects on growth of male nestlings, raise the hypothesis that natural changes in yolk-androgen concentrations with laying order are ultimately due to a difference between the sexes in their yolk-androgen sensitivity and between early- and late-laid eggs in their sex ratio. By re-analyzing previously published data and adding to the analysis data from previously unanalyzed blood samples used for sex determination, we investigated possible sex-specific effects of yolk-androgens in the context of a potential sex-biased laying order in free-living American kestrels. We used a multi-level, mixed model with a Gompertz function to analyze growth of nestlings hatching from a-eggs that were control-treated or in which we experimentally elevated yolk-androgen concentrations shortly after laying to the higher concentrations naturally found in later-laid eggs. We discovered that male nestlings were more susceptible than female nestlings to growth inhibition by yolk-androgen elevation but did not find a bias in sex ratio with respect to laying order. Together, these findings do not support the above hypothesis. However, they are consistent with the hypothesis that sex differences in yolk-androgen sensitivity enable mothers to economically tune reproductive effort to an individual offspring's reproductive value, which can vary more for one sex than the other.