While birds inhale oxygen and expel carbon dioxide through their mouth or nostrils just as mammals do, their respiratory systems are more complex than those of other animals. Birds feature organs called air sacs that connect to their lungs. These air sacs extend into some of the hollow bones of birds and increase the amount of gas inside their bodies at all times.
One big difference between avian respiration and mammalian respiration is that birds do not use a diaphragm to pump air in and out of their lungs. Instead, birds use muscular contractions to move their rib cages, which pumps air through their bodies. Because of their elongated respiratory system, birds store the air in their bodies for two breathing cycles. When a bird inhales, the air it breathed in is not expelled the next time it exhales; instead, the air is expelled the second time the animal exhales as it takes a considerably longer amount of time to travel through the circuit.
The lungs of birds are relatively small, but when combined with the air sacs, the volume of the entire system is significant relative to their body size. In addition to respiration, birds use their respiratory systems to aid in thermoregulation.