Context-dependent effects of feather corticosterone on growth rate and fledging success of wild passerine nestlings in heterogeneous habitat.

Life history theory seeks answers to questions about how suites of traits, like growth rate, body mass and survival, have coevolved to maximize the fitness of individuals. In stochastic environments, individual fitness may be closely linked to environmental conditions experienced early in life. When conditions deteriorate, animals have to adapt physiologically to avoid detrimental effects to growth and survival. Hormones such as glucocorticoids are potentially important mediators of developmental plasticity, although their function is quite poorly understood in free-living animals to date. In this study, we used brood-size manipulation in wild great tits (Parus major) to see whether resource (e.g. food) availability can change feather corticosterone levels, somatic growth and fledging success in nestlings raised in habitats of different quality. Recent studies suggest that feather corticosterone offers a long-term hormonal measure for the main avian glucocorticoid by integrating the plasma levels of corticosterone over the whole nestling period. We showed that feather corticosterone, growth rate and fledging success were significantly affected by the treatment only in coniferous forests where growth conditions had a tendency to be poorer than in deciduous forests. We also found that feather corticosterone was negatively related to fledging success, and this effect was more pronounced in coniferous habitat. Our results suggest that feather corticosterone could offer an important physiological measure for nestling performance, mediated by a context-dependent developmental trade-off between immediate and future survival.