Age-related patterns of yolk androgen deposition are consistent with adaptive brood reduction in spotless starlings

Abstract

Female birds can influence offspring development by adjusting egg size or by a differential allocation of egg resources. Such maternal effects can be expected to be shaped by natural selection, given the costs connected to the allocation of maternal resources. Among egg components, yolk androgens play an important role in affecting offspring life-history traits. Despite their relevance for nestling development, factors accounting for the observed within- and between-clutch variation are still poorly known. By using a cross-sectional sampling approach, we tested the effect of female age, laying order and laying date on the deposition of yolk androstenedione (A4) and testosterone (T), since young and older females could maximize their fitness differently according to breeding conditions. We found a remarkable lack of differences in overall yolk androgen levels across different ages. However, comparing first-year with older females, our results showed that age did not influence yolk T levels at the beginning of the breeding season, whereas at the end, first-year females transferred lower hormone levels than older females. Within clutches, both androgens increased across the laying sequence, suggesting that late nestlings benefit from an increased allocation that could compensate hatching asynchrony. However, when considering A4, we found that, whereas older females always increased A4 levels across the laying sequence, first-year females did not increase it at the end of the breeding season, thus increasing the likelihood of brood reduction when environmental conditions became hardest. These findings suggest that yolk A4 variation may be particularly important at the within-clutch scale, by providing females with a tool to modify nestling hierarchies. The study of maternal effects shows that avian mothers can adjust offspring development to environmental conditions by altering egg composition. The effects of maternal age on such egg allocation process depend on the balance of benefits and costs that affects each age class. We examine if young and older mothers differ in terms of yolk androgen deposition and egg quality, since age may influence offspring fitness, and could represent a costly maternal investment. We found an absence of overall age-dependent patterns in yolk androgen deposition, except when we considered laying order and laying date. In late clutches, when breeding conditions are harsher, 1-year-old females carried out a lower deposition of both resources (yolk mass) and modifiers (androgens) than older mothers did. We suggest that such a pattern would reinforce the effects of hatching asynchrony, favouring brood reduction at the end of the breeding season by first-year females.