The superlative for the highest altitude migrant goes to the Bar-headed Goose (Anser indicus), which has been seen at up to 33,382 feet (10175 m). This bird, which breeds in Central Asia, migrates through the Himalayan range. The air at these heights is so thin that helicopters cannot fly there and kerosene cannot burn. The Bar-head has a slightly larger wing area for its weight than other geese, which is believed to help the goose fly so high.
Audubon Magazine continues:
The yearly migration of these geese is apparently triggered by a biological alarm that rings early enough in the spring for them to miss the summer monsoon season and early enough in the fall for them to miss the worst of winter’s storms. Still, they cast their fates to the wind only with due consideration. Geese poised to take off, for example, may delay their flight if strong headwinds kick up. And when airborne birds get tossed about, they may turn back and land or change altitude in search of better conditions.
Moreover, by using tailwinds, the geese capitalize on weather that could pulverize lesser creatures. “These birds are powerful flappers, not soarers that just glide with the wind,” says M.R. Fedde, an emeritus professor of anatomy and physiology at Kansas State University’s School of Veterinary Medicine, who has conducted laboratory studies of the bar-headed goose’s respiratory system. Partly because their wings are huge, have a disproportionately large surface area for their weight, and are pointed to reduce wind resistance, “they can fly over 50 miles an hour on their own power,” Fedde says. “Add the thrust of tailwinds of perhaps 100 miles an hour if they are lucky, and these birds really move.” Able to gauge and correct for drift, bar-headed geese can even fly in crosswinds without being blown off course. The same powerful and unremitting flapping that helps propel them over the mountains also generates body heat, which is retained by their down feathers. This heat, in turn, helps keep ice from building up on their wings.
What’s the secret to the bar-headed goose’s aerobic success? “First of all, bar-headed geese are birds,” says S. Marsh Tenney, an emeritus professor of physiology at Dartmouth Medical School, whose research on respiratory adaptations to oxygen deprivation includes studies of these highfliers. “And all birds are built for particularly efficient oxygen uptake.” The avian breathing system is uniquely structured. Among its special features are several sacs that temporarily store inhaled air that has passed through the lungs and then send it back through their lungs before it is exhaled. Thus, birds circulate inhaled air through their lungs twice–once more than earthbound mammals do–increasing their opportunities for capturing oxygen.
Additional information can be found via the IUCN Red List of Threatened Species.