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 species because they affect accumulation of fat reserves by nestlings. However, the causal relationship between rearing conditions and post-fledging survival might also be mediated by other mechanisms; for example, the amount and quality of food provided by parents to each nestling might affect development of immune system organs and functions and, hence, the ability of offspring to cope with parasites and pathogens. 2. The hypothesis that parental feeding effort, food quality and brood size 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 intradermal inoculation of a lectin (phytohaemagglutinin) in a large sample of nestlings from unmanipulated broods and broods whose size had been manipulated immediately after hatching. 4. In unmanipulated broods, immune response, body mass and body condition were correlated negatively with brood size and positively with the rate of parental feeding to each offspring. Nestlings in enlarged broods had smaller immune response and body mass, and received less food per capita than those in reduced broods. 5. Broods artificially provisioned with a food rich in proteins showed larger immune response, but not larger body mass, as compared to unprovisioned controls. 6. We conclude that T-lymphocyte cell-mediated immune response as well as body mass is influenced by the level of parental investment and brood size, perhaps via its effect on competition for food, Since T-lymphocytes are fundamental components of avian immunity, and parasites are known to affect survival of their avian hosts, our results suggest a new pathway through which rearing conditions might influence offspring survival.