For humans, transcontinental flight without jet engines, pressurized cabins, and tens of thousands of kilograms of fuel is almost unthinkable. But each year, bar-headed geese fly from Mongolia to India and back, crossing the world’s highest mountains with just their wings and a little extra body fat. Now researchers know just how these 3-kilogram birds make this journey. Rather than flying high for the whole trip, the geese follow the terrain, taking advantage of updrafts to regain altitude as needed.
“The answer comes in the form of clever behaviors for exercise efficiency,” says Terrie Williams, an exercise and environmental physiologist at the University of California, Santa Cruz, who was not involved in the work. These behaviors enabled the birds “to conquer altitudes that man will never achieve without a plane.”
Of the migrating birds, bar-headed geese (Anser indicus) are among the more remarkable. They weigh more than 98% of all birds, yet one has been tracked migrating as high as 7290 meters, an altitude where many humans would require supplemental oxygen to move. The thin air means the birds have to work extra hard to stay airborne and make progress. “We wanted to know how high they flew, what were their flight paths and strategy with respect to the weather conditions, how difficult did they find these journeys, and how much energy did it require,” says Charles Bishop, a zoologist at Bangor University in the United Kingdom.
In 2011 and 2012, he and his colleagues learned through satellite tracking studies that these birds often fly in the very early morning or at night, perhaps to take advantage of the much colder and therefore denser air that allows them to get more lift and thrust for their efforts. In the lab, they nudged geese to run on treadmills in reduced oxygen to simulate high altitudes, which revealed that the birds could keep running at top speed for 15 minutes. Humans would not be able to sustain that pace in such conditions. “They have an exceptional ability to keep exercising in low-oxygen environments,” Bishop says.
To learn more about how this species manages the actual migration, Bishop’s team implanted tubes containing instruments for measuring body temperature, pressure, acceleration, and heart activity into seven geese. The researchers then released the birds into the wild in Mongolia. The readings, some of which were taken every 30 seconds and others of which were taken every 2 minutes, were stored until the researchers recaptured four of the birds a year later. They “provided an unprecedented record of instantaneous physiology in the field,” Williams says. The pressure sensors revealed the altitude, and the researchers translated acceleration data into wingbeat frequency and amplitude. They estimated, based on heart activity, the amount of oxygen used and power created.
Although many migrating birds, such as those that fly between North and South America or between Africa and Europe, climb to a few thousand meters and stay there, taking advantage of tail winds, bar-headed geese fly as if on a roller coaster, following the ups and downs of the ground, Bishop and his colleagues report online today in Science. On average, the geese flew at an altitude of about 4500 meters, but often they changed altitude. For example, one of the four birds whose data was retrieved dropped 1000 meters in 20 minutes, then climbed more than 2000 meters in the next 1.5 hours.
The physiological data explain why. When flying high, the birds flap their wings not only faster, but also more deeply up and down, to stay airborne. The increases in wingbeat frequency exponentially increase the heart rate—which sometimes reaches about 450 beats per minute—and power needed. At lower elevations, where oxygen is more plentiful, the heart rate is much lower—about 300 beats per minute—and the power needed is much less, making descents worthwhile. Even when they were climbing, the birds sometimes didn’t have to work as hard as they do to maintain level flying when high up. They apparently ride wind deflected off the ground to regain lost elevation, Bishop says.
As a result, over the journey the heart rate averages about 330 beats per minute, showing “that most of the time these large birds were flying well within their maximum physiological capacity, despite the extremely difficult conditions,” Bishop notes.
“This study is a real breakthrough for comparative physiology and shows what can be done with physiological sensors in free-living animals,” says Christopher Guglielmo, a physiologist specializing in migration at the University of Western Ontario in London, Canada, who was not involved with the work. “No airplane would fly this way,” he says, but it works for the geese.
Not everyone is sold on the results, however. “It is cool in the basic elements of the study, but greatly overextends what reliably can be interpreted from their data,” says Andrew Biewener, a biomechanist at Harvard University who was not involved with the work. He worries about how the researchers calculated flight energy and power costs, as he’s not sure the other studies they incorporate are really relevant.
Yet the findings reinforce a trend researchers are seeing in other species. “This study, as well as many we have done on animals living on land and in the water, come to similar conclusions: Animals are remarkably adept at finding the energetically most efficient way to move through their respective environments,” Williams says. At the annual meeting of the Society for Integrative and Comparative Biology earlier this month in West Palm Beach, Florida, she described how Weddell seals save energy by gliding as they dive for food and how mountain lions often sit and wait for sneak attacks rather than constantly searching for prey. “Bottom line,” she concludes, “all animals cheat.”
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