Hatching Asynchrony in European Bee-Eaters Merops apiaster

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Abstract
Broods of European bee-eaters Merops apiaster in southern France hatch asychronously; nestling ages within broods range over 2-9 days. Fledging is as, or nearly as, asynchronous as hatching. The last chick to leave fledges about 32 days after the first chick hatches and 28 days after the last, at an age independent of hatching asynchrony. Observations during laying and hatching show that hatching asynchrony results from a progressive increase in incubation during laying. Hatching asynchrony is not correlated with clutch size, date of breeding or age of the mother, but with incubation period of the first egg; synchrony carries the cost of delayed hatching of this egg. Rate of nestling provisioning peaks 3 weeks after hatching and then declines and, during the decline, younger chicks at one nest received less food for their age than older siblings. Chicks aged 3-4 weeks at one nest could monopolize food by their position in the nest burrow, but there was no dominance hierarchy in behaviour controlling access to food. Nestling mortality falls disproportionately on smaller chicks at a colony with high mortality and slow chick growth, and their growth is impaired under all conditions. However, subsequent recruitment does not depend on position in the brood size hierarchy. The pattern of nestling growth and mortality in good and poor conditions supports the brood-reduction hypothesis for the evolution of hatching asynchrony; but the relative timing of laying, hatching and fledging of individual eggs is also consistent with the hurry-up and nest-failure hypotheses. The insurance and peak-load hypotheses can be refuted.