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.

中文翻译：

---

与年龄相关的卵黄雄激素沉积模式与一尘不染的椋鸟的适应性育雏减少一致

雌鸟可以通过调整鸡蛋大小或鸡蛋资源的差异分配来影响后代的发育。考虑到与母体资源分配相关的成本，可以预期这种母体效应会受到自然选择的影响。在鸡蛋成分中，蛋黄雄激素在影响后代的生活史特征方面发挥着重要作用。尽管它们与雏鸟发育相关，但解释观察到的离合器内和离合器间变化的因素仍然知之甚少。通过使用横截面抽样方法，我们测试了雌性年龄、产蛋顺序和产蛋日期对卵黄雄烯二酮 (A4) 和睾酮 (T) 沉积的影响，因为年轻和年长的雌性可以根据育种的不同而最大限度地提高其健康水平使适应。我们发现不同年龄段的总体卵黄雄激素水平显着缺乏差异。然而，将第一年与年长雌性进行比较，我们的结果表明，在繁殖季节开始时，年龄不影响卵黄 T 水平，而在最后，第一年雌性转移的激素水平低于年长雌性。在离合器内，两种雄激素在整个产蛋顺序中都增加，这表明后期雏鸟受益于增加的分配，可以补偿孵化不同步。然而，在考虑 A4 时，我们发现，虽然年长的雌性在整个产蛋序列中总是增加 A4 水平，但第一年的雌性在繁殖季节结束时并没有增加它，因此当环境条件变得最困难时增加了育雏减少的可能性. 这些发现表明，通过为雌性提供修改雏鸟等级的工具，蛋黄 A4 变异在离合范围内可能特别重要。对母体影响的研究表明，鸟类母亲可以通过改变卵子成分来调整后代发育以适应环境条件。产妇年龄对这种卵子分配过程的影响取决于影响每个年龄段的收益和成本的平衡。我们检查年轻和年长的母亲在卵黄雄激素沉积和卵子质量方面是否存在差异，因为年龄可能会影响后代的健康状况，并且可能代表一项昂贵的母亲投资。我们发现卵黄雄激素沉积不存在总体年龄依赖性模式，除非我们考虑产蛋顺序和产蛋日期。在后期，当繁殖条件更加恶劣时，与年长的母亲相比，1 岁的雌性在资源（卵黄质量）和调节剂（雄激素）方面的沉积较少。我们建议这种模式会加强孵化异步的影响，有利于第一年雌性在繁殖季节结束时减少育雏。