Ounce for ounce, a bird in flight requires more energy than a terrestrial mammal. Especially when migrating, birds fly at altitudes where oxygen is in such short supply that no mammal could possibly survive. Birds therefore have evolved a respiratory system that is fundamentally different from the mammalian respiratory system

Like mammals, birds have two symmetrical lungs that are connected to a trachea (windpipe). But here the similarity ends. Mammalian lungs contain many bronchi (tubes), which leads to small sacs called alveoli. Because alveoli have only one opening, air can flow into and out of them, but it can never flow through them to the outside of a lung.

These air sacs fill a large proportion of the chest and abdominal cavity, and also connect to the air spaces in the bones. Two primary bronchi, leading from the trachea, and a number of secondary bronchi, leading from the primary bronchi, feed air into the abdominal air sacs . Some of the secondary bronchi, which spread over the lower surface of each lung, channel air to the anterior air sacs.