Geese use 'roller coaster' strategy to fly over Himalayas

An international team of researchers studying the migration of bar-headed geese in the Himalayan Mountains revealed Thursday that the birds hug the terrain as they fly, riding the peaks and valleys like the hills and dips of a roller coaster.

This unexpected flight pattern, described in the U.S. journal Science, may help the geese conserve more energy than if they were to fly steadily at extreme altitudes over 7,000 meters.

The study, including researchers from Britain, Mongolia, Canada, Australia, Germany and the United States, implanted devices in the geese to monitor their heart rate, abdominal temperature and pressure as well as body movements during their southern migration from their breeding grounds in Mongolia to their wintering grounds in south-eastern Tibet, China or India.

Previously, it was commonly assumed that the geese would fly to high altitudes relatively easily and then remain there during their flights, possibly benefiting from a tailwind.

Instead, the new study showed that the geese tend to fly closest to the ground when traversing the mountains, although this means repeatedly shedding hard-won altitude only subsequently to regain height later in the same flight.

The birds adopt this roller coaster strategy because the decreasing air density at progressively higher altitudes reduces their ability to produce the lift and thrust required to maintain flight, the team said.

The birds would also face the problem of reduced oxygen availability if they were to fly at high altitudes as atmospheric oxygen concentrations fall from 21 percent at sea level to about 10.5 percent at 5,500 meters and about seven percent at the top of Mt. Qomolangma, also known as Mt. Everest.

Wingbeat frequency increased at higher altitudes as bar-headed geese strive to move through the thinner air, the study showed.

An increase in wingbeat frequency would then result in an exponential elevation in heart rates and estimated metabolic power, suggesting that it would cost the birds too much energy to maintain horizontal flight at high altitudes, it said.

The team was also surprised to find that, very occasionally, bar-headed geese were flying in air updrafts created by the mountains.

"This would give them the best opportunity to obtaining assistance from wind that is deflected upwards by the ground ( known as orographic lift), thus, providing additional rates of ascent with either a reduction in their energetic costs or at least no increase," co-author Pat Butler, professor of the University of Birmingham, said in a statement.

While previous studies show that these birds may be capable of flying over 7,000 meters, 98 percent of the new observations show them flying below 6,000 meters.

"Our highest single records were of birds flying briefly at 7290 meters and 6540 meters and seven of the highest eight occurred during the night," said co-author Lucy Hawkes of the University of Exeter, explaining that flying at night would reduce the cost of flight compared to the daytime because the air is colder and denser.

Overall, it's more efficient for the birds to fly low to the ground and to lose and regain altitude, the team said.

Given that energetic costs are thought to increase with body mass and that bar-headed geese are heavier than 98 percent of bird species, "it is particularly impressive that these birds are able to migrate across the world's highest land massif while remaining comfortably within their physiological capabilities," the team wrote.

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