From Wiki ref Bar-headed geese.
The bar-headed goose is one of the world's highest-flying birds, having been heard flying across Mount Makalu – the fifth highest mountain on earth at 8,481 m (27,825 ft)
Surprisingly, despite predictable tail winds that blow up the Himalayas (in the same direction of travel as the geese), bar-headed geese spurn these winds, waiting for them to die down overnight, when they then undertake the greatest rates of climbing flight ever recorded for a bird, and sustain these climbs rates for hours on end, according to research published in 2011
Studies have found that bar-headed geese breathe more deeply and efficiently under low oxygen conditions, which serves to increase oxygen uptake from the environment. The haemoglobin of their blood has a higher affinity for oxygen compared to low-altitude geese.