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Prefledging mass recession in Manx shearwaters: parental desertion or nestling anorexia?

In many species of bird, nestlings undergo a period of mass recession, associated with a large reduction in food provisioning, before they fledge. This has been ascribed to parental desertion, although it could equally result from voluntary limitation of feeding (i.e. anorexia) among chicks. We examined the interactions between Manx shearwater, Puffinus puffinus, parents and chicks, to test whether the prefledging reduction in feeding was controlled by parents, chicks or both. In a cross-fostering experiment, chicks of different ages were exchanged between nests. We predicted that under a parental control model, cross-fostering would have no effect on food provisioning by the parents, but under chick control, the level of provisioning would be adjusted according to the age-specific requirements of the foster chick. Older chicks placed in the nests of younger chicks entered mass recession at an older age and had higher food-provisioning rates than unmanipulated chicks over the 10 days prior to fledging, supporting the parental control model. Younger chicks placed in the nests of older chicks had lower fledging masses than unmanipulated chicks, as a result of lower food-provisioning rates prior to fledging, which also supports this model. However, parents continued to feed younger chicks beyond the date at which their own chick would have fledged. This suggests that chicks also influenced food provisioning, and that mass recession resulted from interactions between a relatively fixed provisioning pattern in parents and age-specific food requirements of chicks.