The ulna (ulna) and radius (radius) together form the forearm and are usually about as long as the humerus. The ulna is the 'heaviest' bone of the two. In gulls, these two bones are relatively slightly longer than the humerus. In petrels usually a fraction shorter and in auks significantly shorter than the humerus. This shortening of the ulna and radius is related to the diving lifestyle and the forces that the wings must withstand. Between the two bones is room for the muscles that control the hand wing. In penguins all wing bones are flattened and are in this respect reminiscent of the internal shape of whale flippers.