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

We manipulated brood sizes to promote different levels of parentaleffort in the common swift (Apus apus). This provided a powerfulmethod for testing hypotheses regarding parental investmentdecisions concerning optimal allocation strategies between parentsand young. Data were analyzed on a visit-by-visit basis regardingchanges in parental and chick body mass, the mass of prey delivered,and the estimated mass of parental self-feeding. Our resultswere consistent with current theory in that food delivery increasedwith brood size, whereas the food received per chick, and hencemean chick body mass, decreased with brood size. Parental bodymass decreased with brood size and increasing parental effortbut recovered quickly during lower levels of chick feeding immediatelybefore fledging, suggesting some short-term cost of reproduction.Parents feeding at the highest level experienced criticallylow body mass and responded by a temporary cessation of chickfeeding. On any one foraging trip, total mass of prey captureddid not differ between brood sizes, but load mass deliveredto the young was negatively related to the amount of estimatedparental self-feeding. Allocation decisions of parents feedingthemselves and their young matched differential allocation theories,but estimated provisioning efficiency of parents at differentbody masses did not suggest any adaptive advantage from parentalmass loss.