Why do many birds have honeycombed bones?

Honeycombed bones - birds fly so well because they have developed skeletons which are especially light and strong. Most of their bones are hollow with the interior webbed or honeycombed across by fine girders of bone to give added strength. They are sometimes called "pheumatic" or air filled bones. A bird's skull is made of thin bone in remarkable contrast to the solid heavy skull of the mammal. The bones of its spine are flexibly connected in the neck strongly bound together in the front part of the body and united at the rear into solid rigid mass. Powerful muscles attached to the breast bone move the wings. The bones in the wings have been reduced in number to provide greater strength. Wings can be used also as propellers. They can be shortened or lengthened by flexing the feathers at the tips can be spread or closed, and the angle of the wings or their parts can be altered. All these adjustments make the aerodynamics of a bird's wings much more complicated than those of an aircraft. Consequently the flight of a bird is more varied and adaptable.