Hatching Asynchrony and Sibling Size Hierarchies in Gulls: Effects on Parental Investment Decisions, Brood Reduction and Reproductive Success

Abstract

We manipulated the within-brood mass hierarchy of Lesser Black-backed Gulls Larus fuscus, controlling for chick quality (related to laying order) by swapping last hatched (c-) chicks in experimental broods with aor bchicks from residual broods to create either synchronous broods (all 3 chicks in the brood approximately the same mass) or asynchronous broods (same within-brood difference in mass between largest and smallest chicks as that of unmanipulated broods), whilst also controlling for variation in characteristics of parental quality (clutch size, egg size and timing of laying). Subsequent chick growth and survival for each brood were monitored in comparison with naturally asynchronous, unmanipulated broods in order to assess the effect of hatching asynchrony per se on chick growth and survival. There were no differences among treatments in fledging success or growth characteristics of surviving chicks. However, chick mortality occurred significantly later in synchronous broods and this was a consequence of the smaller within-brood hierarchical differences in size and body condition of these chicks. Apparently, parents could not regulate resource allocation in synchronous broods, and invested a greater amount of parental investment in chicks that later died. The results indicate that (1) the degree of hatching asynchrony may be optimized with respect to the prevailing food supply and/or female nutritional condition, and (2) the degree of hatching asynchrony is more important than initial within-clutch differences in egg or chick quality in determining the outcome of sibling rivalry over parental investment.