Birds and mammals are both vertebrates (from the same Phylum). Although the specifics of their skeletons differ quite a bit, the overall body plan tends to be the same in both groups. For example, the front limbs (the wings of birds, and the front legs of mammals) have the same basic structure: one large upper arm bone (the humerus, which is shaded red in the figures), two bones in the forearm (radius and ulna), several wrist bones, and then the digits, or phalanges. This module will focus on the humerus, or upper arm bone.