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. 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.
Birds can also pant for prolonged periods without constricting the blood vessels in their brains.
So even when physically taxed, they keep their wits about them. They do everything even better than other birds. Once their blood is stoked with oxygen, it rushes through capillaries that penetrate particularly deep into their muscles. Thus energized, their wings flap with seemingly inexhaustible vigor. Other migratory birds, without the superior flapping, respiratory, and circulatory power of the bar-headed goose, fly closer to the ground.