http://www.nytimes.com/2009/08/04/science/04obgoose.html

For the bar-headed goose, migration is a high-altitude adventure. Spring and fall it flies between Central Asia and India, a route that takes it over the highest mountains in the world, the Himalayas. The bird has been known to reach altitudes of 30,000 feet.

At such heights, the air is so thin that there’s only about a quarter of the oxygen available at sea level. Yet the goose is able to sustain the level of O2 consumption — 10 to 20 times normal — needed for flapping flight.