Climbers struggling the last few steps to the peak of Makalu in the Himalayas have long marveled at the sight of bar-headed geese flying high above to their winter refuge in India. The birds cruise at an altitude of 29,500 feet, nearly as high as commercial aircraft.

For years scientists believed that strong tailwinds and updrafts aided the geese on their journey. A team of researchers led by Charles Bishop of Bangor University in North Wales tested this theory by tracking more than a dozen bar-headed geese harnessed with small backpacks containing satellite transmitters that established their location, speed and altitude.

To their surprise, the researchers discovered that instead of flying in the early afternoon, when heat from the earth can create 12-mile-per-hour updrafts, bar-headed geese consistently fly at night or during early-morning hours, when there is actually a slight downdraft. In a paper published recently in the Proceedings of the National Academy of Sciences USA, the team theorizes that because air is cooler and denser at these times, it allows the geese to generate greater lift. Cooler air also helps to regulate body heat and contains more oxygen, enabling geese to fly even as the air thins at higher levels.

Bishop and his colleagues also were amazed to find that the geese cross the Himalayas in a single day. To fly so far at such a great height, the bar-headed geese must sustain a 10- to 20-fold increase in oxygen consumption. By comparison, lower-altitude birds such as the Canada goose cannot sustain resting levels of metabolism at 30,000 feet. Bigger wings, bigger lungs, a dense network of capillaries surrounding the flight muscle, and hemoglobin that more tightly binds oxygen to the lungs work together to sustain oxygen flow throughout the bird’s circulatory system, including its flight muscle. Improving the understanding of why tissues in bar-headed geese are so adept at taking up oxygen might elucidate human respiration as well.