Maximum Free Glucocorticoids Link Natural Behavior and Stressors in a Wild Bird Population

Glucocorticoids (GCs) regulate suites of traits that can have far-reaching consequences on reproduction and survival. While studies of GC effects on performance are common, results vary widely in their relationship to fitness. Focusing studies on performance metrics that have repercussions for both reproduction and survival may help clarify extended consequences of GC secretion. In this study we evaluated relationships between endogenous GC reactivity, body condition, food availability, and breeding-site departure during storms in a population of white-crowned sparrows early in the breeding season, asking whether GC secretion in response to our capture and handling challenge predicts natural behavioral responses to inclement weather, and whether food availability influences that decision. Our data support a role for GC reactivity in decisions to abandon nascent territories, in that more reactive individuals—those with higher free CORT in response to capture and handling stress—depart sooner during storms. In this case only free GCs (unbound to corticosteroid binding globulin) predict behavior, supporting the hypothesis that free hormone in the plasma is the biologically active fraction. We also suggest a role for food availability in these decisions, as males that located a supplemental food source did not depart during a storm. The very small sample size in the feeding study, however, limits any broad conclusions. All together these data (1) link GC reactivity (free max GC) to a naturally-induced behavior, and (2) support that individual variation in GC elevation has consequences for trade-offs in survival and reproduction.