IMMUNOCOMPETENCE AND NESTLING SURVIVAL IN THE HOUSE MARTIN - THE TASTY CHICK HYPOTHESIS

In altricial birds post-fledging survival is usually positively relate d to nestling body mass. A large number of studies have shown that the latest hatched chick is the more likely to die, even if food is abund ant. Here we suggest that ectoparasites may be a key factor in the evo lution and the maintenance of the establishment of weight hierarchies within broods. We prepose the hypothesis that weight hierarchies withi n broods may be adaptive if the chick in poor condition is the one wit h the least efficient immune system within a nest. In this case parasi tes would preferentially feed on such a ''tasty chick'', because it wo uld allow high reproductive rates for the parasites, without negativel y affecting the survival of the other nestlings. This could prevent en tire nest failure of the brood or allow the other chicks to grow more efficiently. This hypothesis was investigated in a colony of house mar tins Delichon urbica. We predicted that immunocompetence was positivel y correlated with body condition, and that nestlings dying before hedg ing should have lower immune responses when challenged with an antigen . T-cell immune response to an experimentally injected antigen was str ongly positively related to body condition. Non-surviving chicks had l ow body condition and a weak immune response. The implications of thes e results are discussed in the context of the adaptive significance of hatching asynchrony.