How do the lungs of a bird differ from those of a human?

The lungs of birds differ significantly from those of mammal. In addition to the lungs themselves, birds have posterior and anterior air sacs (typically nine) which control air flow through the lungs, but do not play a direct role in gas exchange. They have a flow-through respiration system.

When a bird inhales, air flows in through the trachea to the posterior air sacs, while air currently within the lungs flows into the anterior air sacs. When the bird exhales, the fresh air now contained within the posterior air sacs is driven into the lungs, and the stale air now contained within the anterior air sacs is expelled through the trachea and into the atmosphere. Two complete cycles of inhalation and exhalation are, therefore, required for one breath of air to make its way through the avian respiratory system.

See http://www.nationmaster.com/encyclopedia/Lung for more information.