Baseline and stress-induced corticosterone levels are higher during spring than autumn migration in European robins.

During spring and autumn migrations, birds undergo a suite of physiological and behavioral adaptations known as migratory disposition. The position of migratory seasons within the annual cycle and specifics of environmental conditions in each season could lead to formation of specific regulatory mechanisms of spring and autumn migratory disposition. However, this topic remains largely unstudied. Here we compared corticosterone (CORT) concentration (baseline and stress-induced) in European robins (Erithacus rubecula) captured during seven consecutive migratory seasons on the Courish Spit in the Baltic Sea; >650 plasma samples were analyzed in total. We found that baseline and stress-induced CORT concentrations in free-living robins during spring migration were nearly twice as high comparing to autumn passage. Moreover, the strength of relationship between these two parameters differed between the seasons. In autumn, individuals with elevated baseline CORT level invariably expressed high stress response; in spring, the stress response was more variable. These facts are in line with the hypothesis that spring and autumn migrations are separate life history stages characterized by similar physiological and behavioral adaptations but somewhat different regulatory mechanisms. Further work is needed to understand effects of seasonal differences in CORT concentrations in regulation of migratory disposition in birds.