Corticosterone in territorial male Swainson’s Thrushes varies in relation to forest age but not vegetation cover

Abstract

Glucocorticoids are thought to be related to habitat quality and may provide information about the relative health of individuals. We used a model selection approach to test whether plasma glucocorticoid levels of the Swainson's thrush (Catharus ustulatus) were associated with two attributes that may reflect breeding habitat quality in coniferous plantation forests: stand age and vegetation cover. For baseline corticosterone, the top model included stand age with limited support for models that included vegetation cover. Mean baseline corticosterone for terri- torial male thrushes sampled in mature stands was 78.2 % higher than territorial males sampled in early seral stands. For handling-induced corticosterone, there was limited separation among candidate models, and only one model, containing stand age and Julian day, was better supported than the null model. Despite previous research that has shown hardwood cover is an important component of breeding habitat quality for thrushes, our investigation found limited evidence that vegetation cover was associ- ated with either baseline or handling-induced corticoste- rone levels. Given that early seral and mature conifer forests are characterized by markedly different habitat features, we hypothesize that the observed differences in baseline corticosterone represent either reduced fitness of male thrushes in mature stands or an adaptive adjustment of glucocorticoid levels to match local environmental conditions that equalize fitness across stands of different ages.