The bar-headed goose flies over the Himalayan mountains on its migratory route between South and Central Asia, reaching altitudes of up to 9,000 m. We compared control of breathing in this species with that of low-altitude waterfowl by exposing birds to step decreases in inspired O2 under both poikilocapnic and isocapnic conditions. Bar-headed geese breathed substantially more than both greylag geese and pekin ducks during severe environmental (poikilocapnic) hypoxia (5% inspired O2). This was entirely due to an enhanced tidal volume response to hypoxia, which would have further improved parabronchial (effective) ventilation. Consequently, O2 loading into the blood and arterial Po2 were substantially improved. Because air convection requirements were similar between species at 5% inspired O2, it was the enhanced tidal volume response (not total ventilation per se) that improved O2 loading in bar-headed geese. Other observations suggest that bar-headed geese depress metabolism less than low-altitude birds during hypoxia and also may be capable of generating higher inspiratory airflows. There were no differences between species in ventilatory sensitivities to isocapnic hypoxia, the hypoxia-induced changes in blood CO2 tensions or pH, or hypercapnic ventilatory sensitivities. Overall, our results suggest that evolutionary changes in the respiratory control system of bar-headed geese enhance O2 loading into the blood and may contribute to this species' exceptional ability to fly high.