Final answer:
Birds have small lungs that are accompanied by several air sacs, allowing for a unidirectional flow of air and efficient gas exchange, well-suited for the high oxygen demands of flight.
Step-by-step explanation:
Birds have evolved a specialized respiratory system to meet the high oxygen demands of flight. They have small lungs that are accompanied by several air sacs within their body. Unlike mammals, where air flow is bidirectional, causing a mix of oxygenated and deoxygenated air, birds benefit from a unidirectional air flow system. This is made possible by the posterior air sacs which direct air to the lungs, and then out through the anterior air sacs, creating an efficient counter-current gas exchange system with the blood flow. This system allows birds to efficiently extract oxygen even at higher altitudes where the oxygen concentration is lower, enabling sustained flight.