Quick question about bar-headed geese?

Question

Bar-headed geese fly over the world's highest mountain range, the Himalayas, at heights of 25,000 feet.

What adaptations do you think they have which allow them to fly so high?

Answer 1

Mostly it lies in the ability of the blood to absorb large amounts of oxygen, as I recall the platelets of the blood cells are more numerous than other species, and smaller, giving a huge surface area collectively, for oxygen absorption, this allows flights at much higher altitudes and for a long period of time when oxygen is not available, it can be drawn from storage, in these cells, there is some evidence as well, that some birds have "re-breathing" ability, that is to say, that the birds in question can close their intake apertures, off, and recirculate the air in their lungs by muscular contractions, as a diver can do, by shutting the mouth and swallowing hard, or using the diaphragm, muscles to recirculate internal air, bringing the unused portion of oxygen rich air, in the lungs, into contact with the "alveolar" ( lung sacs) tissue that allows oxygen to be transported across the tissue barrier into the bloodstream...

Answer 2

Their wings are slightly larger than most other birds of their size, so they can get the extra lift from the wind and be carried higher over mountains.