The Himalayan Mountains rise several thousand meters above sea level, yet somehow bar-headed geese (Anser indicus) migrate over them and the Tibetan Plateau every year. Now, researchers tracking these high-flying birds reveal that they ride those peaks and valleys like the lifts and drops of a rollercoaster. This flight pattern helps them conserve more energy than flying steadily at extreme heights—even if it means they repeatedly lose that hard-earned altitude. The findings were published in Science this week.
Bangor University’s Charles Bishop and colleagues implanted data loggers in seven geese as they migrated from their breeding grounds in Mongolia to their wintering sites in southeastern Tibet or India. These devices monitored the birds’ heart rate (an estimate of oxygen consumption and metabolic power), abdominal temperature and pressure to measure altitude, and body acceleration to determine how often they beat their wings.
Researchers have previously assumed that these heavyset geese fly up to high altitudes and then just stay there with the help of a tailwind. But these new data showed that the geese hug the terrain, adopting a rollercoaster strategy when it becomes more difficult to fly. Wingbeat frequency increased at higher altitudes as the birds have to push through thinner, low-density mountain air; decreasing air density reduces their ability to produce the lift and thrust they need. “As even horizontal flapping flight is relatively expensive at higher altitudes, it is generally more efficient to reduce the overall costs of flying by seeking higher-density air at lower altitudes,” Robin Spivey of Bangor University says in a news release.
Flying low to the ground—and having to recoup lost altitude—is simply more efficient, especially in areas where the mountains create updrafts that help minimize their energetic costs. "It worked out that it was eight percent cheaper for them energy-wise if instead, they followed the undulating pattern of the landscape," Bishop tells New Scientist. Night air is also cooler and denser.
While flying, the geese averaged 328 heartbeats a minute, so they’re staying well within their physiological capabilities. Their heart rates increased exponentially with wingbeat frequency, which was very precisely regulated during each flight—typically varying only 0.6 flaps per second. "They are designed with a very high gearing linkage between the movement of the wing and the cardiac output or flow of blood from the heart,” Bishop explains. A small change in wingbeat frequency of 5 percent would result in a elevation in heart rate of 19 percent and a 41 percent increase in estimated flight power.
"The physiology of bar-headed geese has evolved in a number of ways to extract oxygen from the thin air at high altitudes," says Graham Scott of McMaster University. "As a result, they are able to accomplish something that is impossible for most other birds."
Images: Bruce Moffat Photography (top), Nyambayar Batbayar (middle, bottom)