Each year, bar-headed geese migrate over the Himalayas from India, at sea level, to the Tibetan highlands in China and Mongolia. This journey includes an elevation change of more than 26,000 feet in eight to 12 hours.

“They’re the astronauts of the bird world,” said Julia York, a graduate student at the University of Texas at Austin who has been studying bar-headed geese since she was an undergraduate at the University of British Columbia.

Humans who climb the Himalayas have to acclimatize or use an oxygen mask. The bar-headed goose, however, uses oxygen more efficiently. Scientists have known for decades that these geese have an enhanced ability to bind oxygen in their hemoglobin, a process that moves large quantities of oxygen to individual cells .

In the past, experiments have been done on bar-headed geese that were resting or walking on a treadmill. Studies have demonstrated that bar-headed geese have more capillaries around individual cells in their pectoral muscles than barnacle geese and other related species that don’t fly at such high altitudes. Those cells are also dense with mitochondria, which use oxygen to supply energy to the cell .