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Why do birds need more oxygen in their bodies than humans?

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Most likely because their metabolic rates are higher.  Flight takes an enormous amount of energy, and so their rate of breathing, or air exchange, must be very high relative to humans. However, birds do not breathe like humans; our lungs are bidirectional, meaning that fresh air flows into the lungs and stale air flows out. Birds have a unidirectional system, where  only fresh air flows into the lungs, exhalation being accomplished by 9 air sacs that surround the lungs. This makes for an efficient respiratory system which would be critical for flight.  The pulmonary system of birds also provides another link to dinosaurs, who have similar pulmonary structures.  See more at the link:

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