Unique respiration in birds

Flying birds have unique, efficient and special respiration in them as one of their flight adaptations. Unlike human lungs, their lungs are connected to the Air spaces in bones through the Posterior air sacs. During inspiration the fresh air enters their lungs, Posterior air sacs and then the Bone air spaces. During expiration the used air comes out of the lungs; but at the same time the fresh air from the air sacs and the bone spaces enters the lungs (again). Thus there is a constant supply of fresh air to the birds' lungs in both inspiration and expiration!

Birds have this special and wonderful mechanism in their respiration to keep them all active through out their flight.