ABSTRACT Birds migrating through extreme environments can experience a range of challenges while meeting the demands of flight, including highly variable ambient temperatures, humidity and oxygen levels. However, there has been limited research into avian thermoregulation during migration in extreme environments. This study aimed to investigate the effect of flight performance and high altitude on body temperature (Tb) of free-flying bar-headed geese (Anser indicus), a species that completes a high-altitude trans-Himalayan migration through very cold, hypoxic environments. We measured abdominal Tb, along with altitude (via changes in barometric pressure), heart rate and body acceleration of bar-headed geese during their migration across the Tibetan Plateau. Bar-headed geese vary the circadian rhythm of Tb in response to migration, with peak daily Tb during daytime hours outside of migration but early in the morning or overnight during migration, reflecting changes in body acceleration. However, during flight, changes in Tb were not consistent with changes in flight performance (as measured by heart rate or rate of ascent) or altitude. Overall, our results suggest that bar-headed geese are able to thermoregulate during high-altitude migration, maintaining Tb within a relatively narrow range despite appreciable variation in flight intensity and environmental conditions.