Respiratory systemThe lungs and associated structures, whose function is to extract oxygen from the air and to remove carbon dioxide from the body. In birds, because of their inability to sweat, the respiratory system is also important in thermoregulation. The air sac and the pneumatic bone are special features resulting from the adaptation of bird bodies for flight.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver