Cost of reproduction and allocation of food between parent and young in the swift (Apus apus)

Abstract

We manipulated brood sizes to promote different levels of parental effort in the common swift (Apus apus). This provided a powerful method for testing hypotheses regarding parental investment decisions concerning optimal allocation strategies between parents and young. Data were analyzed on a visit-by-visit basis regarding changes in parental and chick body mass, the mass of prey delivered, and the estimated mass of parental self-feeding. Our results were consistent with current theory in that food delivery increased with brood size, whereas the food received per chick, and hence mean chick body mass, decreased with brood size. Parental body mass decreased with brood size and increasing parental effort but recovered quickly during lower levels of chick feeding immediately before fledging, suggesting some short-term cost of reproduction. Parents feeding at the highest level experienced critically low body mass and responded by a temporary cessation of chick feeding. On any one foraging trip, total mass of prey captured did not differ between brood sizes, but load mass delivered to the young was negatively related to the amount of estimated parental self-feeding. Allocation decisions of parents feeding themselves and their young matched differential allocation theories, but estimated provisioning efficiency of parents at different body masses did not suggest any adaptive advantage from parental mass loss.