Because of the large demand of
aerobic respiration for active motions, modification of the lung system
was necessary. Air is inhaled through the windpipe (trachea) and passes
to the air-sacs before flowing through the lungs. The thin-walled air-sacs
act solely as reservoirs; some penetrate the hollow bones. These air-sacs
help in the aerodynamics of the bird; along with the feathers, they help
in streamlining the body, without adding wieght.
Left: The structure of the modern birds lungs.