Asynchronous hatching in a blue tit population: a test of some predictions related to ectoparasites

Abstract

Although much attention has been devoted to understanding the adaptive value of size hierarchies within broods of altricial birds, current hypotheses are the subject of much debate. Recently it has been suggested that parasites could play a role in promoting the establishment and maintenance of a nestling size hierarchy within broods. For example, the "tasty chick hypothesis" stipulates that the last-hatched chick in asynchronously hatching clutches would have the least efficient immune system, resulting in subsequent parasite aggregation on this chick and hence reducing parasitic pressure on the other chicks of the brood. From data collected during four breeding seasons and after experimental removal of the last-hatched chick, we tested three predictions based on variation in chick body characters within broods in a Corsican blue tit (Parus caeruleus ogliastrae) population exposed to high levels of ectoparasites (Protocalliphora spp.). None of the predictions were fully supported and alternative explanations could not be excluded. In particular, there was no evidence for aggregation of parasites on the last-hatched chick or for a beneficial role of this chick with respect to the others, be it "tasty" or not. Thus, our results are not consistent with the hypothesis that mechanisms responsible for size hierarchies within broods have evolved to alleviate the negative effect of parasites on the fitness of their host.