Rearing environment effects on immune defence in blue tit Cyanistes caeruleus nestlings.

Rearing conditions may influence ontogeny and functioning of the immune system. Activation of different mechanisms involved in host disease resistance and their internal regulation can be affected by intrinsic and extrinsic factors influencing development. I investigated how rearing environment can influence associations between humoral and cellular constituents of immune defence in nestling blue tits (Cyanistes caeruleus). The ability to mount a cell-mediated immune response was estimated as a hypersensitivity reaction to phytohaemagglutinin, and the ontogeny of humoral immunity was determined by assessing circulating levels of total IgG in 15-day-old nestlings. Heterogeneity in rearing conditions was evoked by placing nest-boxes in areas differing in habitat structural characteristics, and examining natural variation in nest ectoparasite infestations, hatching date, brood size and brood sex-ratio. Habitat characteristics, parasitism and hatching date may shape associations between different components of the immune system in developing birds. I discuss the effects of rearing conditions on the interaction between different arms of the immune system and the implications for understanding negative correlations within the immune system at the individual and brood level.