The bar-headed goose is much more acrobatic than scientists had previously thought.

The birds have long been heralded for enduring one of the world's most arduous migration routes, reaching heights of 4 miles as they traverse across the Tibetan Plateau and the snow-capped Himalayan mountains on the way to feeding grounds in India.

"It's pretty special that a bird as big as a goose -- a 2 or 3 kilo bird -- can go through such a complex route, through 500 kilometers of some of the highest land on earth," said Charles Bishop of Bangor University, who led an international team which examined how the birds pull of the feat.

A bar-headed goose being released in Mongolia after getting a neck band. Nyambayar Batbayar

Initially, the researchers mostly wanted to understand how they reached such heights.

"I'm interested in how the animals work, how tough was it," Bishop said of his findings, which appeared Thursday in the journal Science.

"What is the physiological mechanism that allows them to perform such an athletic feat at such a high altitude where the air is so thin that they have to flap wings as it turns out a little bit faster and a lot deeper to generate forces from the thin air," he said.

To study the birds, the researchers implanted custom-designed data loggers to monitor pressure-derived altitude, body accelerations and the heart rate of the geese. Then, they collected them when the geese returned to their breeding grounds in Tibet, China and Mongolia.

What they discovered was that, rather than flying steadily at high altitudes, the birds adopt a "roller coaster strategy" throughout their journey -- climbing higher to avoid the mountain peaks but then swooping down to as low as 300 feet and remaining close to the ground.

"You think, oh wow, flying high is easy for them so they are probably flying high quite a lot. So we went out to show they would fly very high and just how high they could go, and could we measure heart rates to show how tough these journeys are," Bishop said of the birds, which he compared to Olympic athletes.

Bar-headed geese. Charles Bishop

"To our surprise, they didn't fly very high very often," he said. "When they did spend time climbing up to get over something, they then would go back down again. There were lots of ups and down and throwing away all this height that they worked so hard to get."

The birds are designed to handle these higher elevations. Their lungs are 25 percent larger than other geese and they have hearts and fly muscles with higher density of blood vessels. But even for them, it can be a difficult slog at these altitudes.

Lower air density reduces the bird's ability to produce the lift and thrust required to maintain flight. And there is the problem of reduced oxygen availability as the atmospheric pressure falls from 100 percent at sea level, with an oxygen content of 21 percent, to about third of that at the top of Mount Everest.

"We know they could fly at 6,000 meters. We can see evidence they are capable of getting enough oxygen," Bishop said. "But the journey would cost them more if they did that. It's easier and cheaper to go down."

They had a few other tricks to save energy, such as flying in the early morning when the air is colder and more dense, "so it's like flying lower."

And when they did have to go higher aloft, the researchers for the first time found the geese got a little help -- from strong updrafts of air that allow them to climb at rates two to three times faster than their normal rate.

"During these moments, it seems likely that the bar-headed geese are flying on the windward side of a valley wall," said University of Birmingham's Pat Butler, a co-author on the study. "This would give them the best opportunity to obtaining assistance from wind that is deflected upwards by the ground, thus, providing additional rates of ascent with either a reduction in their energetic costs or at least no increase."

Andrew Farnsworth, project leader of BirdCast at the Cornell Lab of Ornithology, who was not involved in the research, called it an "exciting study for migration biologists as well as for the public at large with general interest in understanding feats of animals."

"Crossing the Himalayas is an amazing feat, and this story adds a great deal to our understanding of the situation," Farnsworth said in an email. "Of particular interest is the notion that these birds change altitude with much greater frequency than previously presumed, but the costs of doing so are well within the physiological constraints of the birds doing it. In fact, as with other situations in which birds need endurance and speed, they probably have energy to burn."

Farnsworth said the work aligns with other research on the migration strategies of species like thrushes.

"The more we look at these migration feats, the more we see that birds are using particular strategies to achieve them -- and that actually, these become less feats and more regular strategies that birds during migration," he said. "Whereas the crossing of the Himalayas at very high altitude always seemed like a mammoth feat, now we know some important new information about it -- that the costs to maintaining high altitude flight over the whole mountain range is greater than anticipated, and that the reality of making drastic altitudinal changes and tracking terrain, and doing so without burn-out, so to speak, is perhaps an even greater feat."