Bird bone. Coloured scanning electron micrograph (SEM) of part of the jaw socket from a bird skull. The complicated structure of a jaw joint includes spaces for the attachment of muscles, tendons and ligaments. This allows the bones of the joint to move independently (articulate) while remaining connected. In birds, the large jaw bones have been reduced in size and covered by keratin (like in an antler or fingernail) to form the beak.