Birds cross Himalayas

A bar-headed goose on the wing.

Photo: Nyambayar Batbayar

When bar-headed geese migrate across the spine of the Himalayas, they follow the terrain, roller-coaster style, rather than fly a straight-line path from ridge to ridge. This flight path features far more climbing, yet a study published today in Science shows that it’s considerably more efficient than the straight-line course.

To look at how these geese make the long, high-altitude flight between India and Mongolia, Charles Bishop, a senior lecturer at the University of Bangor in the United Kingdom, and a group of collaborators implanted (and later removed) devices that tracked flights of about 15 hours each. (The entire transit of the mountains typically takes 50 to 70 hours.)

The best measure of metabolic activity — and energy output — is oxygen consumption, which cannot be measured on a flying bird. To assess how the birds were powering their flight, the research instrument measured:

Map and relief cross-section of South Asia shows the migratory flight of bar-headed geese. The high-flyer soars south from the Mongolian steppe, over the peaks of the Tibetan plateau, to the lowlands of southern India, and returns north.

Map by The Why Files, adapted from L. A. Hawkes et al., 2012.

The field research team, photographed in Mongolia.

Photo of team by Nyambayar Batbayar

My heart, my oxygen

In previous lab work with bar-headed geese, Bishop found that oxygen consumption changed according to the square of the change in heart rate. For this study, he extended the relationship backward to wing beat: On average, a 5 percent rise in wing beat increased heart rate by 19 percent and oxygen consumption by 41 percent.

Thus a small change in wing beat has major consequences for metabolism and power output.

Other calculations, based vertical acceleration and wingbeat, independently confirmed the birds’ exertion level. “We know that our approximation of body power was reasonable … because it correlated with a totally independent variable, heart rate, which is correlated with oxygen consumption,” Bishop told us. “So power input and output correlated nicely.”

The studies of energetics explained the goose’s surprising preference for the roller-coaster strategy. Even though they did more climbing, they exerted less energy than they would have needed on a course with far less climbing.

Why?

The answer, Bishop says, lies in the drop in air pressure and density with altitude, which makes each wing stroke less efficient. By staying as low as possible, the geese save energy and also breathe denser air that contains more oxygen.

Air pressure could also explain the surprisingly large amount of climbing done at night or in the early morning. “It’s maybe 20° Celsius colder, and that makes the air denser. Staying in denser air reduces the flight cost because it’s easier to generate lift and thrust.”

To take the high road, or the low? Bar-headed geese opt for a ground-hugging flight across mountainous terrain, despite the additional climbing of their roller-coaster route.

Diagram by the Why Files, adapted from Charles M. Bishop et al¹.

Here’s a surprise: Although there were times when the geese seemed to “coast” (flap with less force, particularly during tailwinds and updrafts), there was no evidence of gliding — flying without flapping. “There were some dramatic cases where there was a massive climb rate with either no increase in heart rate, or a decrease, while they were climbing three times faster than the normal rate,” Bishop says.

Group of bar-headed geese in Mongolia

Photo: Charles Bishop

Those updrafts are stronger near the ground, Bishop adds, supplying another physical reason to stay low. He says the updraft advantage is roughly equal to the benefit that geese derive from flying in a Vee formation, which results from pressure changes that trail off the leading bird’s wing. (Bar-headed geese fly in Vees, Bishop says, but it’s not known whether they use Vee formation for their high-altitude mountain transits.)

Bar-headed geese “are specialists in high-altitude flight,” Bishop says, and have been tracked up to 7,290 meters (4.5 miles). Nonetheless, instead of staying high until the mountains are passed, “they throw the altitude away to reduce the overall cost.”

We asked about the evolutionary implications of this study, and he told us, “This does confirm that they behave in a very intelligent way. Nature once again has shown us that animals tend to move toward the most economical way of doing things that gives the best long-term survival option.”