Maternal androgens in black-headed gull (Larus ridibundus) eggs: consequences for chick development

We tested the hypothesis that mother birds counterbalance the negative effe cts of hatching asynchrony for later-hatched chicks by increasing the yolk androgen concentrations in consecutive eggs of their clutch. In doing so. t he) may adaptively tune each offspring's competitive ability and, thus, gro wth and survival. However, evidence in support of this hypothesis is contra dictory. The yolk concentrations of maternal androgens in the eggs of black -headed gulls increase significantly with the laying order of the eggs in a clutch. We experimentally tested tile functional consequences of this incr ease on chick development under natural conditions by injecting eggs with e ither an oil or androgen solution. We created experimental clutches in whic h androgen levels either stayed constant or increased with laying order whi le controlling for differences in egg quality by using only first-laid eggs . We then compared development, growth and survival between these broods. A ndrogen treatment enhanced embryonic development because androgen-treated e ggs hatched half a day earlier than controls: while their size at hatching was similar to oil-treated controls. Androgen treatment did not increase ch ick survival, hut it enhanced growth.,Androgen-treated, third-hatched chick s had a higher body mass and longer legs than third-hatched chicks that hat ched from oil-treated eggs. At the same time, growth of first chicks which M ere all oil treated was reduced by tile presence of two androgen-treated siblings, suggesting that yolk androgens enhance the competitive ability of later-hatched chicks. Our results support the hypothesis that transfer of different amounts of androgens to the eggs of a clutch is a mechanism by wh ich mothers maximize their reproductive output.