Bar-headed geese fly up and over breathtakingly high passes in the Himalayas in just eight hours without rest or the help of tailwinds, satellite tracking shows.

Researchers had previously known that the geese undertake one of the highest-altitude migrations in the world, flapping their way over snowy peaks through thin mountain air that leaves human adventurers gasping for breath and struggling to walk. But they didn't know how fast the birds could climb over 6,000-metre ridges from their breeding grounds in Mongolia to their winter refuges in India.

"We were fairly amazed that they do this in a single flying period," said Charles Bishop, a biologist at Bangor University in the U.K. Bishop led the study, which was published Monday in Proceedings of the National Academy of Sciences.

The birds mostly chose to fly at night and in the early morning, when there was little wind and the weather was calm. (Nyambayar Batbayar ) The study was a collaboration among researchers around the world, including Graham Scott and Bill Milsom at the University of British Columbia.

Researchers already knew that bar-headed geese have larger lungs, a faster breathing rate, blood that binds better to oxygen and more capillaries in their muscles compared to other geese — adaptations that help them perform athletic feats at high elevation.

Nevertheless, the researchers thought the geese might still rest along their journey or rely on favourable winds that blow mainly in the afternoon — the way many small birds do.

"They didn't do that," Bishop said. "They seem to have the capacity to do these climbs under their own power without any assistance."

Instead, the birds chose to fly at night and in the early morning, when there was little wind and the weather was calm.

Bishop said the researchers were initially surprised by that, but they realized that the geese were behaving in a way that was most likely to help them minimize the risk of encountering dangerous storms.

More oxygen, more lift

Temperatures are also colder in the early morning, making the air denser. Denser air carries more oxygen and increases the lift that the birds' wings can generate as they flap, suggesting that the birds may rely on those effects as well.

To monitor the birds' flight, researchers attached satellite tags that transmitted information about the birds' speed, position and altitude every hour. They compared that to data from the nearest weather stations, which were on Mount Everest about 100 to 150 kilometres away.

The second phase of the ongoing study involves monitoring the birds' heart rates and body temperatures as they fly - a part that heavily involves the UBC researchers, Bishop said.

The researchers are also interested in knowing the maximum altitude that the birds can fly to.

In this particular study, they reached about 6,000 metres — about 2,800 metres lower than the peak of Mount Everest.

The paper includes an anecdote from Lawrence Swan, a naturalist who accompanied Sir Edmund Hillary on a 1960 expedition to the Himalayan peak Makalu, the fifth highest in the world.

Swan reports that on a "cold and still night in early April," as he stood next to the Barun glacier, he heard the honking of bar-headed geese. The timing is consistent with the researchers' findings, but they have so far not heard any similar anecdotes from modern-day climbers, Bishop said.

"We would be very interested in hearing from mountaineers who have physically seen these birds at these altitudes."