IMMUNOCOMPETENCE OF NESTLING BARN SWALLOWS IN RELATION TO BROOD SIZE AND PARENTAL EFFORT

1. Intra-brood competition and parental feeding effort are considered important determinants of survival of offspring in altricial bird spec ies because they affect accumulation of fat reserves by nestlings. How ever, the causal relationship between rearing conditions and post-fled ging survival might also be mediated by other mechanisms; for example, the amount and quality of food provided by parents to each nestling m ight affect development of immune system organs and functions and, hen ce, the ability of offspring to cope with parasites and pathogens. 2. The hypothesis that parental feeding effort, food quality and brood si ze affect immunocompetence of nestlings was tested for the first time in the barn swallow, Hirundo rustica, Linnaeus. 3. The intensity of T- lymphocyte cell-mediated immune responsiveness was evaluated after int radermal inoculation of a lectin (phytohaemagglutinin) in a large samp le of nestlings from unmanipulated broods and broods whose size had be en manipulated immediately after hatching. 4. In unmanipulated broods, immune response, body mass and body condition were correlated negativ ely with brood size and positively with the rate of parental feeding t o each offspring. Nestlings in enlarged broods had smaller immune resp onse and body mass, and received less food per capita than those in re duced broods. 5. Broods artificially provisioned with a food rich in p roteins showed larger immune response, but not larger body mass, as co mpared to unprovisioned controls. 6. We conclude that T-lymphocyte cel l-mediated immune response as well as body mass is influenced by the l evel of parental investment and brood size, perhaps via its effect on competition for food, Since T-lymphocytes are fundamental components o f avian immunity, and parasites are known to affect survival of their avian hosts, our results suggest a new pathway through which rearing c onditions might influence offspring survival.