Sibling competition and the risk of falling out of the nest

In birds, sibling competition encompasses several activities, one of which is jostling for position, that is, competing for the location in the nest where parents predictably deliver food items. We hypothesized that nestlings that compete by jostling for position may fall out of the nest either accidentally or because siblings push each other to reduce brood size. This hypothesis predicts that in a competitive environment needy nestlings trade-off the benefit of being fed against the cost of falling out of the nest. As a first attempt to evaluate this hypothesis, we experimentally manipulated the number of young per brood in the colonial Alpine swift, Apus melba. Nestlings fell out of their colony more frequently when reared in enlarged than in reduced broods. Because brood size manipulation affects not only the number of young per nest but also their body condition, we analysed an extended data set to disentangle these two factors. This analysis showed that, independently of brood size, nestlings in poor condition and those reared in broods where sibling differed markedly in weight were more likely to disappear from the colony. Nestling disappearance also occurred predominantly in nests close to the colony entrances. Although nestling swifts can wander in the colony and become adopted in neighbouring nests, we found no evidence that wandering per se increased the risk of falling out of the colony. Our study therefore highlights a novel cost of scramble competition.