Within-brood size differences affect innate and acquired immunity in roller Coracias garrulus nestlings

The immune system has a crucial importance determining animal health and survival. Its maintenance and activation are costly and usually trade with other physiologically costly functions. In birds, number and size of siblings in a nest are likely to determine the development of immunity. Indeed lower immunocompetence is expected in large than in small broods. Moreover, in asynchronous breeders, siblings are expected to differ in immunocompetence because asynchrony produces marked size hierarchy within-broods. Here we studied the effect of environmental conditions at the nest, chick sex and size, and natural mass differences among siblings due to hatching asynchrony on the development of the innate and acquired immune systems in the threatened non-size dimorphic asynchronous breeder European roller Coracias garrulus . Constitutive innate immune function was estimated by using a new technique proposed by Matson et al. (2005) . Natural Antibody (NAb) and Complement levels, innate immunity, varied with the mass difference between each chick and its heaviest sibling. NAb levels were higher in late-hatched siblings compared to early-hatched ones, indicating that the smallest offspring in each brood has the most developed innate immune system. This relationship was independent of the nest environment. The heterophil/lymphocyte (H/L) ratio, which may indicate the level of stress, was higher in the heaviest siblings of each brood. In addition, the H/L ratio and white blood cell count (WBC) of nestlings, measures belonging to both the innate and acquired arms of the immune system, were related to conditions suffered in the nest. Our results may be explained by differential allocation of resources by female rollers as a way to improve the survival of the whole brood.