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.

在众多物种中，个体同胞间的相对年龄是其适合度变异的重要诱因。本研究以蓝山雀（Cyanistes caeruleus）为对象，发现其年龄层级主要源于亲鸟在整窝卵完全产下前便启动孵化的行为——即最后产下的卵会比早产卵的孵化时间更晚。然而，在对孵化行为进行统计学控制后，研究显示晚产卵的孵化速度反而快于早产卵的卵，从而降低了孵化异步性的程度。我们在早、晚产卵当日，于不同鸟巢间互换早产卵与晚产卵的卵，以此控制差异化孵化的影响，结果发现晚产卵卵更快的孵化速度这一现象依然存在。孵化更早的雏鸟体重更重，成功离巢的概率也更高；而来自互换实验卵的雏鸟，其生长与存活模式也与该规律相符。卵的重量仅能解释该变异中的一小部分，剩余变异则应归因于卵的成分差异。上述结果与“产卵序列中不同卵的内在差异可缓解年龄相关层级带来的影响”这一观点一致。此外本研究还发现，不同窝卵间的产前发育速率存在变异，且该变异主要源于环境因素，而非遗传因素。