What progressive features in the structure of the respiratory system have arisen in birds, in contrast to reptiles?

Birds have air sacs in their respiratory system. They are enlargements of part of the bronchi of the lungs. In flight, in birds, part of the air, when inhaled, enters the air sacs. There is no gas exchange in these bags and therefore oxygen-rich air is retained. On exhalation, air from these bags enters the lungs, where gas exchange takes place.