Coping With Extreme: Highland Eurasian Tree Sparrows With Molt‐Breeding Overlap Express Higher Levels of Corticoserone‐Binding Globulin Than Lowland Sparrows

Abstract

In birds, suppressed expression of stress-induced corticosterone (CORT) during pre-basic molt is generally thought to reflect a physiological trade-off in self-maintenance. And reduced CORT during breeding in extreme environments may maximize reproductive success and optimize their fitness. Highland Eurasian tree sparrows (Passer montanus) are known to express significantly higher stress-induced CORT levels during the pre-basic molt stage. Here, we show that these highland sparrows are characterized by a life history strategy of molt-breeding overlap, with higher corticosterone-binding globulin (CBG) levels favoring molt and breeding consistent with fitness optimization on the Tibetan Plateau. These unique behavioral and physiological strategies reflect natural selection under strong evolutionary pressures in extreme high-altitude environments.