Final answer:
Birds have small lungs that are assisted by air sacs which enable unidirectional airflow and efficient gas exchange, aiding their ability to fly.
Step-by-step explanation:
Birds have small lungs that are accompanied by several air sacs. These air sacs are crucial as they store inhaled air and then push it into the lungs, enabling the lungs to remain filled with oxygenated air at all times, even during exhalation. This is important for birds due to the high energy requirements of flight, necessitating a constant and efficient supply of oxygen. Unlike mammals where air flows back and forth, creating a dilution of oxygen concentration, birds have a unique respiratory system in which airflow through their lungs is unidirectional. This, coupled with the counter-current exchange system with the blood in the air capillaries, allows for a highly efficient means of gas exchange. The lightweight bones of birds, filled with air and connected to these air sacs, also contribute to their flying capabilities by reducing their overall weight.