Basically, I saved the best for last, because birds have the most interesting and dynamic respiratory system of all animals. Indeed, they need a high-performance system to help them meet the high oxygen consumption demands of flight and to fly at high altitudes where the partial pressure of oxygen is low.
Birds have a pair of lungs connected to the outside air by a single trachea.
Comparatively speaking, those features are pretty much the extent of the anatomical similarity between the avian and mammalian respiratory system. The air sacs act as a series of bellows to move air through the lungs in one direction, rather than bidirectionally. What's more, air moves continuously through a bird's tertiary bronchi during both the inspiratory and expiratory phase. Thus, unlike mammals, a bird has virtually no deadspace air. There are no anatomical one-way valves in a bird's respiratory system to keep the air flowing unidirectionally.