The bar-headed goose crosses the Himalayas and has been recorded flying at over 7,000 metres.
Why it is true
At that altitude the air holds less than half the oxygen found at sea level. These geese have larger lungs, haemoglobin that grabs oxygen more easily and more capillaries in their muscles, and they fly mostly at night, when the air is colder and denser.
Source: Science (Bishop et al., 2015)
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