No. In birds, air sacs are not lungs. Birds have lungs specialized for gas exchange, and they also have separate air sacs that store and move air through the respiratory system. OpenStax Biology notes that oxygenated air diffuses across the surface of the lungs into the bloodstream, while air flows in one direction from the posterior air sacs to the lungs and out through the anterior air sacs. That flow-through design makes gas exchange highly efficient for flight, but the exchange itself happens in the lungs—not inside the air sacs.
