The respiratory system of birds differs from that of mammals. Birds have relatively small and poorly elastic lungs with air capillaries instead of alveoli in which air passes in one direction. The lungs get ventilated by distension and compression of air sacs. The larynx of birds doesnÃ¢â‚¬â„¢t play a role in sound-producing, instead, the syrinx is the vocal organ. The diaphragm has no muscles and is built out of 2 thin septa.