Natural Climbers: Insights from Avian Physiology at High Altitude.

High altitudes are physiologically challenging: the hypobaric hypoxia, cold, and increased ultraviolet radiation mean humans ascending to high altitude faster than they acclimatize risk life-threatening illnesses. Despite such challenges, birds can thrive at high altitudes and some even complete metabolically costly migrations across the world's highest mountain ranges. We outline the aspects of avian anatomy and physiology that confer advantages at each level of the oxygen transport cascade and compare them with those of human and nonhuman mammals.