Editor's note: An October 2012 study in the Proceedings of the Royal Society B found that bar-headed geese follow valleys through the mountains, which keeps them below 18,000 feet nearly all of the time. However the birds do make occasional forays to higher altitudes, a feat that still can't be explained, according to the study.

The world's highest flying bird is an Asian goose that can fly up and over the Himalaya in only about eight hours, a new study finds.

The bar-headed goose is "very pretty, but I guess it doesn't look like a superathlete," said study co-author Lucy Hawkes, a biologist at Bangor University in the United Kingdom. (See bird pictures.)

In 2009, Hawkes and an international team of researchers tagged 25 bar-headed geese in India with GPS transmitters. Shortly thereafter, the birds left on their annual spring migration to Mongolia and surrounding areas to breed.

To get there, the geese have to fly over the Himalaya—the world's tallest mountain range and home to the tallest mountain on Earth, Mount Everest, which rises to 29,035 feet (8,850 meters).

The researchers that found the birds reached a peak height of nearly 21,120 feet (6,437 meters) during their travels. The migration took about two months and covered distances of up to 5,000 miles (8,000 kilometers).

The birds made frequent rest stops during the migration, but they appear to have flown over the Himalayan portion of their journey in a single effort that took about eight hours on average and that included little or no rest. A similar intense climb could kill a human without proper acclimatization, Hawkes said.

"If you've ever seen a goose sitting on a lake, take-off is quite an energetic thing, so it may be [energetically] cheaper to keep going than to keep sitting down and taking off again—and they may not want to delay getting over the mountains," Hawkes said.

(Read about National Geographic research tracking migration patterns of animals around the world.)

Birds' Bodies Built for High Flying

Even more impressive, the birds completed the ascent under their own muscular power, with almost no aid from tail winds or updrafts.

"Most other species that we've identified as high-altitude flyers usually get there by soaring and gliding up," said study leader Charles Bishop, also a biologist at Bangor University.

By contrast, the bar-headed goose reaches such lofty heights by flapping vigorously, if not gracefully.

"Geese tend to honk a lot as they fly," co-author Hawkes said. "We don't think of them as the most elegant of migrants."

The birds have evolved numerous physiological adaptations—many of which are not so obvious—to help them complete their migrations.

"They have all these internal morphological tweaks that make it possible," Bishop said.

For example, past studies have shown that the geese have more capillaries and more efficient red blood cells than other birds, meaning their bodies can deliver more oxygen to muscle cells faster.

The geese's flight muscles also have more mitochondria—energy-producing structures inside cells—than their fellow fowl.

Another trick in the birds' arsenal: hyperventilation. Unlike humans, bar-headed geese can breathe in and out very rapidly without getting dizzy or passing out.

"By hyperventilating, [the birds] increase the net quantity of oxygen that they get into their blood," Hawkes explained.

Geese Older Than the Hills?

Before the new study, many scientists had thought the geese were taking advantage of daytime winds that blow up and over mountaintops. But the team showed the birds forgo the winds and choose to fly at night, when conditions are relatively calmer.

"They're potentially avoiding higher winds in the afternoon, which might make flights more uncomfortable or more risky," said Bishop, whose study appeared May 31 in the Proceedings of the National Academy of Sciences.

The birds could potentially head east or west and fly around, rather than over, the mountain range, but this would add several days to their trip and would actually use up more energy.

"I guess it's like when you're trying to get in the grocery store and there's a steep set of stairs and a really long [wheelchair] route. You have to work out which one you can be bothered to do on a particular day," Hawkes said.

Another hypothesis about why the geese choose to fly over rather than around the Himalaya is that the birds have been doing so for millions of years—long before the mountains reached their current heights.