Within seasons and among years: when are corticosterone levels repeatable?

Hormones play a central role in integrating internal and external cues to help mediate life-history decisions as well as changes in behavior and physiology of individuals. Describing the consistency of endocrine traits within and among individuals is an important step for understanding whether hormonal traits are dependable predictors of phenotypes that selection could act upon. However, few long-term field studies have investigated the individual consistency of hormonal traits. Glucocorticoid hormones mediate homeostatic responses to environmental variation as well as stress responses to acute, unpredictable disturbances. We characterized the repeatability of plasma corticosterone concentrations in two species of free-living passerines across multiple years. We found repeatability in baseline corticosterone concentrations in both sexes of great tits (Parus major) and in female tree swallows (Tachycineta bicolor) within the breeding season but no repeatability of this trait among seasons or across years. Stress-induced levels of corticosterone were only assessed in great tits and were not repeatable in either sex. Our data suggest that in line with their function in mediating responses of individuals to longer-term and acute demands, both baseline and stress-induced plasma corticosterone concentrations are rather plastic traits. However, individuals may differ in their degree of trait plasticity and hence in behavioral and physiological responses to a variety of organismal challenges.