Data from: Hatching hierarchy but not egg-related effects governs behavioral phenotypes in gull chicks

In many bird species that practice parental care, siblings often compete for resources and care provided by their parents, although their strategies differ according to hatching rank and condition. Differences in offspring strategies are generally attributed to hatching order and maternal effects, which are difficult to separate because these effects are often correlated. For example, third-hatched chicks of large gull species receive more egg testosterones and corticosterone, which influence early behavioral patterns. In this study, we carried out a cross-fostering experiment with first- and last-laid eggs of the yellow-legged gull (Larus michahellis) to test whether the within-brood variation in behavioral strategies for competing with siblings and coping with stress are due to maternal effects or to hatching order. Chicks hatched in the last position within the experimental brood emitted more chatter calls to attract parents’ attention, were less prone to respond to warning of danger, and had a lower breathing rate while restrained than first-hatched chicks. Egg laying order did not affect chick behaviors or breathing rate. Thus, we concluded that the different behavioral strategies of chicks were determined by their posthatching experience and not by the original egg position within the clutch. Last-laid eggs were smaller and chicks from those eggs grew slower than chicks from first-laid eggs. Independently of the original laying order, chicks that hatched first in the experimental brood grew faster than their siblings. Overall, our results indicate that behavioral strategies of chicks are plastic and influenced by their early social experience.

在诸多存在双亲抚育行为的鸟类物种中，同胞间往往会围绕双亲提供的资源与照料展开竞争，且其竞争策略会随孵化顺位与自身状态的不同而存在差异。后代竞争策略的差异通常被归因于孵化顺序与母体效应，但由于这两类因素往往存在相关性，因此很难将二者区分开来。例如，大型鸥类的第三只孵化出的雏鸟会获得更多卵睾酮与皮质酮，这类激素会影响雏鸟早期的行为模式。本研究针对黄腿鸥（Larus michahellis）的首枚与末枚产卵开展交叉抚育实验，旨在验证同胞间竞争与应对压力的行为策略在同窝内的差异，究竟是源于母体效应还是孵化顺序。相较于首只孵化的雏鸟，实验同窝中最后孵化的雏鸟会发出更多啁啾鸣叫声以吸引双亲注意，对危险预警的响应意愿更低，且在被束缚时的呼吸频率更低。产卵顺序并未对雏鸟的行为或呼吸频率产生影响。据此我们认为，雏鸟不同的行为策略由其孵化后的经历决定，而非产卵窝内的初始产卵位置。末枚产卵的卵体积更小，其孵化出的雏鸟生长速度慢于首枚产卵孵化的雏鸟。无论初始产卵顺序如何，实验同窝中首只孵化的雏鸟生长速度都快于其同胞兄弟姐妹。总体而言，本研究结果表明，雏鸟的行为策略具有可塑性，且会受到其早期社会经历的影响。