Pneumatic Bones: Avian Skeletal Adapta tion   Pneumatic bones are a fascinating evolutionary flight adaptation in birds. The   bones of birds, including Archeopteryx and Pterosaurs, but not bats, are hollow rather than filled with blood-forming or fatty tissues like the bones of other vertebrates. Absence of these tissues from bird bones results in overall lightening of the skeleton and reduces the weight that must be launched into the air. The hollow air-filled (pneumatic) bones of birds are probably an ancestral character of the archosaur lineage, not a derived character of birds. People misinterpret pneumatic bones as hollow structures, but actually they are highly sophisticated providing an incredible strength-to-weight ratio, to withstand the stresses of flight. Structure of Pneumatic Bones Hollow tubes have greater strength than solid rods of the same mass and the avian skeleton has taken advantage of this principle to produce a supporting structure that is not o...