The effect of laying and hatching order on the timing and asynchrony of hatching

Asynchronous hatching is believed to result from incubation starting before laying is completed and leads to a hierarchy in chick size, with detrimental effects on the small, last-hatching chick. If chicks have some control over the timing of hatching, however, they might be able to compensate at least partly for the hatching spread. In this study on lesser black-backed gulls, Larus fuscus, we examined whether the timing of hatching varied with laying order. To separate their effects, we manipulated the hatching order relative to the laying order within broods of three chicks. We exchanged eggs between nests, so that chicks from first-laid eggs hatched last, and chicks from last-laid eggs hatched first. When hatching last, chicks from last-laid eggs took significantly less time to hatch than chicks from first-laid eggs. However, hatching duration did not differ between chicks from first- and last-laid eggs when hatching first. Hence, only offspring from last-laid eggs had the ability to accelerate hatching, and they did this only when their siblings had already hatched. Accelerated hatching might benefit the last-laid offspring by reducing hatching intervals, but it also resulted in low hatchling mass. As this ability to accelerate hatching was not found in first-laid offspring, differences in intrinsic egg quality may be responsible for hatching duration. Differences in egg quality, however, arise from differential allocation of resources across the laying sequence by the parent and may hence suggest a parental strategy to alleviate the detrimental effects of within-brood hierarchies on the last-hatching chick.