Parent–offspring conflict during the transition to independence in a pelagic seabird

The transition to independence is a critical period of development and a focus of parent–offspring conflict over the optimum level of parental care, but there is continuing uncertainty over how much this transition is influenced by parents or offspring. We experimentally cross-fostered Manx shearwater chicks differing in age by 10–14 days and tested two predictions: 1) food-provisioning rate in the period leading up to fledging is related to the duration of parental care rather than to the age of chicks; 2) parents protect themselves from exploitative offspring by becoming insensitive to begging behavior of chicks over the period leading up to fledging. We also examined whether fledging age was under endogenous hormonal control or influenced mainly by parents. Switching chicks had no effect on fledging age, which was mainly controlled by an internal mechanism linked to a marked and rapid increase in corticosterone secretion, with no difference among treatments in the timing or magnitude of this increase. In contrast, parents reduced their frequency of food delivery according to the number of days elapsed since they started provisioning, regardless of age of chicks and despite younger foster chicks having lower body condition and begging more intensely than older foster chicks or controls. These data provide clear experimental evidence of parent–offspring conflict over parental feeding frequency in late chick development and hence chick body condition at fledging. As predicted, parents resolved this conflict in their favor by responding much less to begging over the period prior to fledging than at earlier stages of chick development.