Bone is relatively hard for its density, although it is brittle and may snap on impact, creating what is medically called a fracture. The reason for the good material characteristics in bone is that it is not solid calcium, but instead a mesh, the density of which may vary at different points in the bone. Thus bird bones are generally less dense than mammalian bones.
Bones are generally tubular in structure, and the hollow space in the middle is filled with marrow. The marrow is where blood cells are produced and thus critically important to healthy blood. At the ends are connections to other bones at joints which are lubricated and sometimes padded by softer tissue called cartilige.