Brood reduction in response to manipulated brood sizes in the common swift (Apus apus)

Abstract

Following a brood size manipulation experiment on the common swift (Apus apus) in which different levels of parental effort were created, brood reduction occurred in all five nests manipulated to four chicks and in two of the five manipulated to three young. This provided an opportunity for looking in detail at the parental investment decisions concerning allocation strategies between parent and young during the process of brood reduction. The data recorded here were analysed on a visit-by-visit basis regarding changes in parental and chick body masses, the mass of prey delivered and the estimated mass of parental self-feeding. The results show that food delivery rates did not increase in proportion to the number of chicks in the broods. This reduction in delivery rates per chick resulted in lower chick masses and ultimately in the death of one or more chicks in the larger broods. The resulting low parental body masses for adult birds feeding larger broods suggested that these parents could not have raised all their young without risking their own survival. There was a tendency for parents from nests that experienced brood reduction to feed themselves instead of allocating most of the food gathered to the young. After brood reduction, parents regained lost body mass and their young fledged at masses only slightly lower than normal. The differential allocation decisions regarding parental self-feeding which resulted in brood reduction were largely responsible for keeping the parents in good enough condition to care for the surviving nestlings. Therefore, this study clearly demonstrates that brood reduction can operate through the differential allocation of food between parent and young in a way that can have adaptive consequences for the survival of parents and their young.