The bar-headed goose flies at altitudes of up to 9000m on its biannual migration over the Himalayas. Despite the severe hypoxia at these elevations, bar-headed geese must sustain high rates of oxygen transport to support the metabolic costs of flight. Based on theoretical modelling we predicted that the physiological traits with the greatest influence over oxygen transport during hypoxia would be a heightened capacity to breathe, a high affinity of the blood for oxygen, and an enhanced capacity for oxygen diffusion in the flight muscle. We subsequently found that bar-headed geese had an enhanced poikilocapnic hypoxic ventilatory response, employing a more effective breathing pattern (reduced dead space ventilation), dramatically improving arterial PO2 compared to other birds. This enhanced HVR was not due to a greater hypoxic sensitivity but to a reduced sensitivity to hypocapnia. Barheaded geese have relatively large lungs and, as with all birds, a thin pulmonary diffusion barrier. Their blood has been shown to have a higher oxygen affinity than that of other birds, which we hypothesize can be dramatically assisted by cooling in the lungs at altitude (to assist loading) and warming in the exercising flight muscle (to assist unloading). We found that, compared to closely related geese, the flight muscles of the barheaded goose contained more type IIa fibres, a higher mitochondrial volume density, an increased capillary density, and a higher proportion of mitochondria in sub-sarcolemmal locations. At the cellular level we found no differences in oxidative capacity, no differences in metabolic enzyme activity and no differences in oxygen kinetics. Finally, while it remains possible that these birds may take advantage of updrafts and wind currents to assist them in crossing over some of the world’s highest mountains, preliminary evidence suggests otherwise. Ongoing studies are designed to determine the true metabolic costs of this amazing feat and to measure physiological changes during free flight at altitude.