The major limb bones of birds are hollow with supportive internal struts, seemingly to make them lighter; however, the bones of a bird's leg are often heavier than those of other vertebrates of similar sizes. Therefore the structure is such to provide support, which they may need due to the stresses that running or landing may lead to.
The wings of birds have evolved from the basic plan of vertebrates. The humerus attaches to the scapula located on the thorax. The ulna and the radius attach to the other end of the humerus. Modified hand bones compose the remainder of the limb. The carpometacarpus is the equivalent to an elongated palm of the hand, much like the tarsometatarsus is equivalent to an elongated arch of the foot. The digits extend off of the carpometacarpus to support the alula, which controls passing airstreams so that more lift can be produced, and primary flight feathers, which are also supported by the ulna and carpometacarpus.