The environment can affect individual performance directly via resource availability or indirectly through resource allocation among competing fitness components, such as body growth and maintenance activities related to short-term survival. Corticosterone (CORT), the main glucocorticoid in birds, may be an important mediator of energy allocation to different organismal functions, but its effect on the plasticity of fitness-related traits has rarely been investigated at different ontogenetic stages. Here, we evaluated the role of baseline and stress-induced CORT on nestling development of wild great tits (Parus major) under different growth conditions and at different developmental stages. We found that nestling mass and condition were positively related to stress-induced levels of total CORT and free CORT in the middle of nestling stage but negatively related at the prefledging stage. Our results also showed that stress-induced levels of total CORT can be used as a proxy for bioavailable free CORT. No significant relationship of body mass or condition in either stage was shown with levels of baseline CORT or stress-induced CORT-binding globulin capacity. None of these age-specific relationships depend on brood size manipulation. We conclude that the effects of glucocorticoids on nestling performance are stage specific. The ability to secrete CORT may have beneficial effects on nestling development in the middle of the nestling stage, while it can be adaptive to keep CORT levels at optimal ranges before fledging to give priority to growth-related processes that are important for long-term fitness.
Keywords: fitness; free corticosterone; glucocorticoids; great tit; growth; trade-off.