Bar-headed geese (Anser indicus) migrate thousands of kilometres twice a year from their breeding grounds in Mongolia, northern China and the Tibetan Plateau to their wintering grounds in India and back again. During this migration, bar-headed geese have to fly across the highest mountains in the world, the Himalayas, where the amount of oxygen is a fraction of that available at sea level. These birds have been spotted flying at high altitudes by mountain climbers, but these accounts can be inaccurate. To determine more about these incredible birds and their arduous migration, Lucy Hawkes from the University of Bangor, UK, along with a team of international collaborators used GPS satellite transmitters and reported their findings in a recent issue of Proceedings of the Royal Society B.

Hawkes and her colleagues attached GPS satellite transmitters to 91 bar-headed geese captured in India, China and Mongolia, in order to track their migration both to and from their breeding grounds. First, the team used the GPS data in combination with land elevation data to determine whether bar-headed geese performed their remarkable migration through the lower mountain passes or whether they were flying over mountain summits. Hawkes and colleagues found that the birds' migration was on average 3000 km and it took about 47 days. Most geese travelled in areas below 6000 m, which are some of the lowest elevations available in this area, indicating that most geese choose the low mountain passes during their migration. However, 10 birds flew higher than 6000 m, with one goose flying at a record 7290 m. The researchers suggested this was possible because the flight was recorded during the night, when air temperatures are colder and the air is denser, conditions that assist flight at high altitude and increase available oxygen.

Flying is a very metabolically demanding task and bar-headed geese are active flyers that do not use gliding. The researchers therefore wanted to determine whether the birds used tailwinds during their migration to help reduce energy expenditure and oxygen needs in an already oxygen-limited environment. Hawkes and her colleagues used their GPS data in combination with modelled weather data to answer this question. Contrary to expectations, the geese did not appear to use tailwinds during their southbound migration to their wintering areas as the wind speeds and directions were no different when the birds were flying from those when they were stationary. Bar-headed geese migrating northbound to their breeding areas also did not use tailwinds. These geese chose to fly when wind speeds were significantly lower than those when they were stationary. The researchers have previously suggested that flying during low wind conditions is safer and allows the birds more control over flight without interfering winds.

Despite bar-headed geese choosing the lower mountain passes during their biannual migration, these birds are still performing a remarkable feat. Even at these lower elevations the available oxygen is only half that at sea level, and the birds' energy demands are increased by actively flying. Despite this, they do not seem to take advantage of tailwinds. The endurance flight of bar-headed geese at these heights is incredible when compared with the performance of humans, who can only accomplish mild exercise at these altitudes even with acclimatization. It goes to show that the sky isn't the limit for the bar-headed goose!

L. A.
P. J.
D. C.
P. B.
W. K.
S. H.
, et al. 
. (
The paradox of extreme high-altitude migration in bar-headed geese Anser indicus
Proc. Biol. Sci.