Birds respire by a method of crosscurrent flow, ie: flow at a 90 degree angle. There are three sections involved in respiration. These are the anterior air sacs (interclavicular, cervicals, and anterior thoracics), the lungs, and the posterior air sacs (posterior thoracics & abdominals). It takes a bird two full breaths (inhaling and exhaling), to cycle air through.
The air flow through air sacs and lungs is as follows:
* First inhalation: air flows through the trachea and bronchi into the
posterior air sacs.
In birds, air flows in only one direction. Because of this, birds are able to diffuse more oxygen into their blood. Unlike humans and other mammals, there is no mixing of oxygen rich air and carbon dioxide rich air. Thus, the partial pressure of oxygen in a bird's lungs is the same as the environment. This is also why you would more likely see a bird on Mount Everest, than say a mouse.