Baseline corticosterone is positively related to juvenile survival in a migrant passerine bird

1. Glucocorticoid hormones are an integral part of the vertebrate stress response, and theoretical models argue for a link between glucocorticoid levels and individual fitness. The cort-fitness hypothesis posits that elevated levels of baseline glucocorticoids are reflective of an individual in poor condition and with a reduced likelihood of survival. Surprisingly, this hypothesis remains virtually untested for the juvenile life-history stage, a period that is often characterized by high mortality rates. 2. To address this issue, we explored whether glucocorticoid levels just prior to fledging were related to survival during the juvenile period in the Swainson's thrush (Catharus ustulatus), a short-lived, temperate-breeding passerine bird. Over 2years, we blood-sampled nestling thrushes to quantify glucocorticoid levels and then used radio telemetry to assess whether individuals died or survived to emigrate from the study area. Finally, we measured vegetation characteristics at the nest and at locations used by individuals during the juvenile period to quantify the relative importance of habitat features and glucocorticoid levels on survival. 3. Predation was the leading cause of death, and overall juvenile survival rate was 34.6%. We found that survival was positively associated with baseline corticosterone and, to a lesser extent, size-corrected body mass and date of fledging. Contrary to expectations, we found no evidence that the amount of vegetative cover at the nest site or at locations used during the juvenile period was associated with survival. 5. Although we cannot completely rule out the cort-fitness hypothesis, our data appear to support the cort-activity hypothesis and suggest that elevated baseline corticosterone levels in juvenile thrushes may be linked to enhanced post-fledging survival via increased locomotor activity that promotes foraging, more effective escape from predators or both.