Bar-headed geese rise and fall with the terrain below them when they migrate, scientists report in the Jan. 16 Science. This roller coaster flight pattern saves the birds energy, even though they must repeatedly climb to recover the altitude they lose at each dip in the topography.
An international team of researchers tracked altitude, along with wingbeats and heartbeats, in migrating bar-headed geese (Anser indicus) as they traveled over the Himalayas and Tibet. Flying along a high-altitude path is more work than staying low with occasional climbs, the researchers concluded.
The findings are a surprise, considering that geese are known for straight, fast flight, says Steven Portugal, a comparative physiologist at the University of London. “Geese are not renowned for flight maneuverability or sophistication,” he says.
With altitude, air becomes less dense. At an elevation of 5,500 meters, the air contains half as much oxygen as at sea level. Bar-headed geese have adaptations — such as higher lung capacity than other geese and more blood vessels in the flight and heart muscles — that allow them to reach altitudes of more than 7,000 meters.
“Before, the understanding was that the real challenge is the oxygen concentration,” says Gil Bohrer, an environmental engineer at Ohio State University. “Not enough attention was paid to the physical challenge of flight in thin air.”
As the birds migrated from Mongolia to India, tiny data loggers implanted in their abdomens recorded atmospheric pressure to monitor the birds’ altitude. The instruments also recorded the birds’ heart rates and how their bodies moved in response to each flap from the wings, indicating the rate at which their wings were beating.
The researchers used GPS coordinates from a previous study to compare the birds’ dips and climbs in altitude with the topography of the land.
The birds typically stayed within 100 meters of the ground, dipping whenever the terrain dropped and rising when it became higher. “They regularly throw away this altitude that they just worked hard to get,” says Charles Bishop, a coauthor of the study.
The researchers also saw a stronger link than expected between how often the geese beat their wings and their heart rate. A 5 percent increase in wing beat frequency translated to a 19 percent boost in heart rate and a 41 percent rise in oxygen consumption. “Small changes in wing motion are costing them a lot of energy,” says Bishop, a zoologist at Bangor University in Wales. “The thinner air means they must work harder to generate the airflow and the forces they need to generate lift and thrust…. The roller coaster strategy is actually the energetically best strategy.”
At lower altitudes, the geese are less likely to be buffeted by strong winds. In a few cases, the researchers noticed that the birds would ascend very quickly without a corresponding increase in their heart rate. This indicated that the birds were taking advantage of strong updrafts. The researchers also speculate that flying low allows the birds to use gentler updrafts that dissipate with height.
“We know they’re well adapted to high flights,” says Bishop. But “just because they’re capable doesn’t mean they’re doing it all the time.… Their lives are more complicated than that.”
Portugal says he would have liked the research to address whether flying low comes with its own risks. “Does it bring them into closer range for aerial attacks from birds of prey?” he asks.