There are plenty of creatures that make long migrations, but the travels of the bar-headed goose (Anser indicus) are particularly arduous: This bird spends its winters at sea level in India and its summers in central Asia, crossing the Himalayas twice a year. To discover just how bar-headed geese accomplish this feat, scientists led by Bangor University in Britain attached satellite transmitters to birds before their spring (northward) and fall (southward) migrations. (The study appears in PNAS.)
The researchers had thought that the geese might be taking advantage of upslope tailwinds that blow from mid-morning to early afternoon. Instead, the geese fly during the night and early morning, climbing to altitudes of 4,000 to 6,000 meters (13,000 to 20,000 feet) and crossing the Himalayas in just 7 to 8 hours in the spring, and 3 to 5 hours in the fall.
If humans tried the goose’s journey, they might experience dizziness or altitude sickness or even die. Not that it’s easy for the geese, though, which also have to deal with thinner air—which makes flapping flight more difficult—and less oxygen. But the bar-headed geese have several adaptations that help them deal with these conditions, such as a greater density of capillaries that supply their muscles, hemoglobin in their blood that is better at taking up oxygen than in other bird species, and larger lungs than other waterfowl.
And by traveling at night and in early morning, the geese are likely able to take advantage of cooler air temperatures, which result in denser air, and calmer winds, thus avoiding the turbulent storms that can occur in the Himalayan afternoon. “As a consequence,” the scientists write, “they can maintain maximum safety and control over their flights, while optimizing lift production and oxygen availability.”