09-Jun-2011 - Bar-headed geese: The Most Extreme Migration on Earth?


High flyers. Bar-headed geese (Anser indicus) make remarkably demanding annual migrations over the Himalayas.
Credit: Nyambayar Batbayer; (inset, upper right) Courtesy of Lucy Hawkes/Bangor University
English article

Way up in the Himalayas, where thin air and low oxygen pressure hinder speech and movement, weary mountaineers have observed bar-headed geese (Anser indicus) honking away as they ascend powerfully overhead. Every year the geese make an epic migration from sea level in India up over the immense mountain range to their summer breeding grounds in Central Asia. A new study shows that they do it quickly and under their own steam, without the help of
upslope tailwinds thought to loft them over the sky-high peaks. The discovery, argues the study’s lead author, makes the birds’ annual journey “the most extreme migration on Earth.”

In recent years, researchers have illuminated a number of adaptations that enable the bar-headed goose’s migratory feat. These include proportionally bigger lungs than many other bird species, a better supply of oxygen to the muscles and heart, denser capillaries, and hemoglobin that carries more oxygen. Even so, most experts have thought the birds couldn’t make the trip without the help of winds that rush predictably up the mountains during the day.

The northbound geese typically made the trip from sea level over mountain passes of up to 6000 meters in just 7 or 8 hours at speeds of 64.5 kilometers per hour. They also logged the highest sustained climbing speed known from any bird species, of just under 1.1 vertical kilometers per hour. 

Most surprising was that the geese completed most of their journeys not during the day with the uplifting winds at their backs, but during the night or early morning, when headwinds were likely, according to data from a Mount Everest weather station. The researchers think the cooler night and early-morning temperatures, which would help dissipate body heat and increase oxygen availability, may be more helpful than any tailwind assist. The geese might also prefer calmer conditions for safety and maneuverability.

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