Bird Lungs Birds have unique lungs because of their high demand for oxygen. Bird lungs are open-ended, leading to and from special air sacs. The lungs are constantly inflated, accepting airflow in only one direction. Depending on the type of bird, he may have seven or nine air sacs extending into his thigh bones, the bones between his elbow and shoulder, his vertebrae and his skull. The bird's complicated respiratory system requires two cycles -- inspiration, expiration, inspiration, expiration -- to circulate air through his entire respiratory system, compared to the single cycle mammals require. However, a bird's respiratory system is more efficient because it moves more oxygen with each breath. Aren't you glad you asked?
I've had birds and never knew that. I know that their respiratory systems are delicate...you gotta watch mold and fumes.