Position in the laying order has sex-specific consequences for reproductive success in adult black-headed gulls

Mothers who produce multiple offspring within one reproductive attempt often allocate resources differentially; some maternally-derived substances are preferentially allocated to last-produced offspring and others to first-produced offspring. The combined effect of these different allocation regimes on the overall fitness of offspring produced early or late in the sequence is not well understood, partly because production order is often coupled with birth order, making it difficult to separate effects of pre-natal maternal allocation from those of post-natal social environments. In addition, very little is known about the influence of laying order on fitness in later-life. In this study, we used a semi-natural captive colony of black-headed gulls to test whether an offspring’s position in the laying order affected its early life survival and later life reproductive success, independent of its hatching order. Later-laid eggs were less likely to hatch, but among those that did, survival to adulthood was greater than that of first-laid eggs. In adulthood, the laying order of females did not affect their likelihood of breeding in the colony, but male offspring hatched from last-laid eggs were significantly less likely to gain a breeding position than earlier-laid males. In contrast, later-laid female parents hatched lower proportions of their clutches than first-laid females, but hatching success was unrelated to the laying order of male parents. Our results indicate that gull mothers induce complex and sex-specific effects on both the early survival of their offspring and on long-term reproductive success through laying order effects among eggs of the same breeding attempt.

单次生殖事件中产出多胎的雌性个体通常会差异化分配资源：部分母体来源的物质会优先分配给最后产出的后代，其余则分配给首批产出的后代。这类差异化分配策略对序列中早产出与晚产出后代的整体适合度（fitness）所产生的联合效应，目前仍未得到充分阐释。部分原因在于产出顺序常与出生顺序绑定，使得研究者难以区分产前母体资源分配效应与产后社会环境效应。此外，学界对产卵顺序（laying order）对后代后期生活适合度的影响所知甚少。
本研究以半自然圈养的黑头鸥种群为实验对象，旨在探究在排除孵化顺序（hatching order）干扰的前提下，后代在产卵序列中的位置是否会影响其早期生存与后期繁殖成功率。晚产卵的孵化成功率更低，但在成功孵化的个体中，其发育至成体的存活率高于首批产出的卵所孵化的后代。成年后，雌性子代的产卵顺序并不会影响其在种群中繁殖的概率，但从最后产的卵中孵化出的雄性子代，相比早产卵孵化的雄性个体，获得繁殖位的概率显著更低。与之相反，晚产卵孵化的雌性亲代，其窝卵（clutch）的孵化比例低于早产卵孵化的雌性亲代；而雄性亲代的产卵顺序则与孵化成功率无关联。
本研究结果表明，黑头鸥亲代通过同一次繁殖事件内的产卵顺序效应，对后代的早期生存与长期繁殖成功率产生了复杂且具有性别特异性的影响。