Sibling Growth Patterns in Great tits: Does Increased Selection on Last-hatched Chicks Favour an Asynchronous Hatching Strategy?

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In birds, competitive abilities of siblings in relation to their sex and the magnitude of hatching delay are still poorly understood. We compared the sex-specific growth of the last-hatched, competitively disadvantaged chicks with that of synchronously hatched chicks in two successive years. Sons exhibited higher growth rates than daughters in a year with delayed onset of breeding, and this sex-related difference was more pronounced among the asynchronously hatched chicks. Females apparently do not selectively allocate more resources to the last-laid eggs because neonatal body mass and the growth rate of asynchronously hatched chicks did not differ between years, despite the fact that in one of the years, asynchronous chicks hatched from replaced eggs taken randomly from other nests and not from eggs laid last by the incubating female. Among chicks that survived, moderately asynchronous siblings grew at a lower rate than synchronous ones, whereas no difference in mass gain was revealed between synchronous and strongly asynchronous siblings irrespective of their contrasting competitive abilities. We suggest that selection of the fittest asynchronous chicks may improve the overall quality of asynchronous broods, thus favouring the maintenance of asynchronous hatching strategies in variable environments.

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