Geese that migrate from Mongolia to India and back every year, flying over the towering Himalaya Mountains, have evolved a "roller coaster" flight pattern to conquer the high-altitude barrier the mountains represent, researchers have found.

Rather than flying to an altitude high enough to clear Himalayan peaks and just staying at that height, bar-headed geese follow the terrain, flying close to the terrain and moving up and down as the topography dictates, scientists have discovered.

"It was thought for a long time that they might actually be flying over the peak of Everest itself," says Doug Altshuler, a zoologist at the University of British Columbia in Vancouver. "Some of the records of early climbers on Everest claimed that when they were climbing on Everest they actually saw the bar-headed geese flying overhead."

While scientist have briefly tracked the geese at 24,000 feet, that's not as high as Everest, nor do they remain as such altitudes for long, the researchers have found.

Tracking devices implanted in seven of the birds by researchers at Bangor University in Britain, recording their altitude, heart rate and other measurements, showed the bar-headed geese don't maintain any single altitude for the duration of their migratory flights but rather climb and descend, matching the contours of the terrain beneath them.

That flight strategy likely allows the birds to conserve their energy as much as possible during their long journeys, the researchers say.

Because the low density of the air at higher altitudes requires the geese to expend more energy to generate lift, descending whenever they can and climbing again only when necessary is the most efficient use of their energy, they say.

Oxygen is also more plentiful at the lower altitudes, they add.

In addition, flying lower allows the geese to take advantage of any updrafts they encounter.

"By staying close to the ground they can maximize those opportunities," says Bangor biologist Charles Bishop. "It's smart, isn't it? Nature seems to always select animals to do things economically, and do it the best way."

Flying close the ground would also make it easier for the birds to occasionally stop for a drink, and being closely to terrain landmarks could be helping them navigate, he says.

In addition to yielding the altitude data, the study showed the geese preferred to fly at night or in the very early morning.

That's likely because the colder air at those times, being much denser, allows them to get more lift and thrust out of their wing beats, Bishop and his colleagues report in the journal Science.