Abstract
Feather corticosterone (fCORT) acts as a reliable indicator of avian physiological responses to environmental stressors. However, variation in fCORT across taxa and ecological contexts, combined with limited knowledge of the stress physiology of molt migrants (individuals moving from breeding to specific molting grounds) has left population-level stress patterns in many species poorly understood. We compared fCORT concentrations in the globally endangered Scaly-sided Merganser (Mergus squamatus, SSME) between its two remaining breeding populations exposed to different stress regimes, while accounting for molting habitat variation inferred from stable isotope analyses. By controlling for the generally higher fCORT concentrations in molt migrants, population-level variation was primarily driven by habitat-specific stressors. Elevated fCORT levels in the Russian population are indicative of heightened physiological stress, whereas the comparatively low fCORT levels observed in the Chinese population may reflect habituation to chronic disturbance. Meanwhile, the divergence in fCORT between populations may be further mediated by population-specific factors, such as migration-related energetic demands and habitat conditions. These contrasting stress profiles highlight the importance of adopting population-specific approaches in conservation planning. Our findings reinforce the growing recognition that physiological indicators offer a powerful framework for assessing population-level responses to differing habitat conditions. Moreover, given the widespread occurrence of heterogeneous molt-movement strategies among birds, integrating molt-related movement ecology into conservation physiology can improve our ability to identify vulnerable subpopulations, refine stress-based monitoring tools, and design management actions that better accommodate intra-specific diversity across avian taxa.