CHIN, E.H.*; LOVE, O.P.; WILLIAMS, T.D.; Simon Fraser Univ., Burnaby, BC, Canada; Simon Fraser Univ., Burnaby, BC, Canada; Simon Fraser Univ., Burnaby, BC, Canada: Sex-specific effects of brood size manipulation on offspring immunocompetance

Previous studies in birds have revealed that experimentally increasing brood size decreases the quality of individual offspring, due to factors such as intra-brood competition, food quality and conflict for available parental resources. One such measure of offspring quality is cell-mediated immunocompetance, which has been shown to be lower in chicks reared in larger brood sizes compared with average brood sizes, affecting survival. In sexually dimorphic species where growth is especially important for the larger sex, the potential trade-off between growth and immune function may be exaggerated when resources are limited by a large brood size. Potentially then, in these species, immunocompetance may play a role in regulation of optimal brood size. To test this hypothesis, we examined cell-mediated immunity in a wild colony of European starlings (Sturnus vulgaris) in which males exhibit higher growth rates and larger structural size than their female siblings. We manipulated brood size (reduced, average and enlarged broods) and examined cell-mediated immunity in chicks near fledging. We predicted that male offspring in larger brood sizes would exhibit not only lower immune response than female siblings, but lower immune response than male offspring in reduced and average sized broods. Results will allow for the examination of inter-sexual differences in physiological strategies which may potentially play a role in the evolution of optimal brood sizes in sexually-dimorphic birds.