Data from: Intra-clutch differences in egg characteristics mitigate the consequences of age-related hierarchies in a wild passerine.

The relative age of an individual's siblings is a major cause of fitness variation in many species. In Blue tits (Cyanistes caeruleus) we show that age hierarchies are predominantly caused by incubation pre-clutch completion, such that last laid eggs hatch later than early laid eggs. However, after statistically controlling for incubation behavior late laid eggs are shown to hatch more quickly than early laid eggs reducing the amount of asynchrony. By experimentally switching early and late laid eggs between nests on the day they were laid we controlled for the effect of differential incubation and found that the faster hatching times of late laid eggs remains. Chicks that hatched earlier were heavier and had higher probability of fledgling, and chicks that hatched from experimental eggs had patterns of growth and survival consistent with this. Egg mass explained a small part of this variation, but the remainder must be due to egg composition. These results are consistent with the idea that intrinsic differences between eggs across the laying sequence serve to mitigate the effects of age-related hierarchies. We also show that between-clutch variation in prenatal developmental rate exists and that it is mainly environmental in origin rather than genetic.

个体同胞的相对年龄是诸多物种适合度变异的主要诱因之一。针对青山雀（Cyanistes caeruleus），本研究证实，其年龄层级主要由一窝卵全部产下前即启动的孵化行为所导致——晚产卵的孵化时间晚于早产卵。然而，在通过统计学方法控制孵化行为的干扰效应后，研究发现晚产卵的孵化速度反而快于早产卵，这降低了孵化不同步的程度。本研究通过在产卵当日将早产卵与晚产卵在不同鸟巢间进行互换实验，控制了差异化孵化的影响，结果证实晚产卵更快的孵化速度这一特征依然存在。孵化更早的雏鸟体重更重，出飞概率也更高；来自实验互换卵的雏鸟，其生长与存活模式也与此相符。卵重仅能解释该变异中的一小部分，剩余变异则应归因于卵的内在成分差异。上述结果与“产卵序列不同的卵之间存在固有差异，可缓解年龄相关层级带来的负面影响”这一假说一致。本研究还证实，不同窝卵之间的产前发育速率存在差异，且该差异主要源于环境因素，而非遗传因素。