High-altitude champions: birds that live and migrate at altitude.

High altitude is physiologically challenging for vertebrate life for many reasons, including hypoxia (low environmental oxygen); yet, many birds thrive at altitude. Compared with mammals, birds have additional enhancements to their oxygen transport cascade, the conceptual series of steps responsible for acquiring oxygen from the environment and transporting it to the mitochondria. These adaptations have allowed them to inhabit a number of high-altitude regions. Waterfowl are a taxon prolific at altitude. This minireview explores the physiological responses of high-altitude waterfowl (geese and ducks), comparing the strategies of lifelong high-altitude residents to those of transient high-altitude performers, providing insight into how birds champion high-altitude life. In particular, this review highlights and contrasts the physiological hypoxia responses of bar-headed geese (Anser indicus), birds that migrate biannually through the Himalayas (4,500-6,500 m), and Andean geese (Chloephaga melanoptera), lifelong residents of the Andes (4,000-5,500 m). These two species exhibit markedly different ventilatory and cardiovascular strategies for coping with hypoxia: bar-headed geese robustly increase convective oxygen transport elements (i.e., heart rate and total ventilation) whereas Andean geese rely predominantly on enhancements that are likely morphological in origin (i.e., increases in lung oxygen diffusion and cardiac stroke volume). The minireview compares the short- and long-term cardiovascular and ventilatory trade-offs of these different physiological strategies and offers hypotheses surrounding their origins. It also draws parallels to high-altitude human physiology and research, and identifies a number of areas of further research. The field of high-altitude avian physiology offers a unique and broadly applicable insight into physiological enhancements in hypoxia.