Corticosterone unlikely indicates departure readiness in migratory European robins

During migration, birds must repeatedly decide when to terminate stopover and resume migratory flight. Such decisions greatly affect their overall migration success and, ultimately, fitness. Departure probability increases with higher levels of energy reserves, positive wind support and progression of the migration season. However, our knowledge of the endocrine mechanisms underlying this process is still incomplete. To evaluate the possible role of baseline corticosterone (CORT thereafter) in regulation of migratory departure, we analyzed the relationships between CORT levels and factors associated with departure probability in European robins (Erithacus rubecula) blood sampled at two nearby stopover sites during six autumn and six spring migration seasons. Contrary to predictions, in both migration seasons we observed higher CORT levels in lean birds, which are assumed to have low departure motivation. We found no indication that CORT concentrations increased in fat individuals captured around sunset, presumably shortly before departure. In parallel with departure likelihood, CORT levels increased towards the end of migration season in autumn, but there was no seasonal trend in spring, when motivation to continue migration is higher. Finally, CORT levels were positively related to tailwind component in autumn, whereas the opposite trend was observed in spring. In summary, our results provide mixed support for the hypothesis that CORT indicates departure readiness in songbirds. We propose that the observed relationships are more consistent with metabolic functions of CORT. Our study highlights the value of multiyear data and seasonal comparisons in studies of the endocrine regulation of such complex behaviors as departure decision.