In 1953, a climber witnessed a bar-headed goose passing by in the Mount Everest at the elevation of nearly 9 kilometers. It’s nearly 2 kilometers higher than other animals’ flight altitude. Why could they fly so high?

Scientists found out later that bar-headed geese have a strong ability to combine hemoglobin and oxygen. Comparing to other birds, bar-headed geese have more blood capillary around their pectorales cells. By experiment, the scientists found out that the bar-headed geese will slow down their metabolism and heart rate, and cool their blood down in some way to make the combination of hemoglobin and oxygen tighter so that their blood carry more oxygen.