Birds' lungs work differently from other animals. For our lungs, the air goes in and then out. Our lungs have lots of tiny sacks called alveoli that inflate and deflate like tiny balloons. This is not tremendously efficient, as often not all of the air is exhaled with each breath, and half of the energy is used for exhaling without delivering any more oxygen.

For birds, the air always flows through their lungs in the same direction. As they inhale, the air goes into an air sack, then through thin, tube-like structures in their lungs, into another air sack, and then is exhaled. Fresh air goes through the lungs as they inhale and as they exhale. This is much more efficient, delivering more oxygen with less effort. The extra oxygen keeps their muscles going, letting them fly farther and faster.