To meet the high oxygen demand of flight, birds have the most efficient respiratory system of any vertebrate. It differs from the respiratory system of mammals in several aspects. Air sacs are thin sacs continuous with the lungs but not involved in gas exchange. They are present within the body cavity and extend into many pneumatic bones. The connect directly to primary and secondary bronchi.
Lungs are spongy in structure, rather than sac-like.
This allows very efficient extraction of oxygen from air. Mice were comatose, sparrows were able to fly normally. Mice and sparrows had similar body mass, similar metabolic rates, and similar blood oxygen affinity, so the difference is most likely explained by the increased efficiency of oxygen extraction. No syrinx is present in vultures, ostriches, Mute Swan, some storks. Most nonpasserines have only 2 extrinsic muscles associated with the syrinx.