The body of the bird, having only two legs, rests on a single line of gravity. The external shape of a bird is very different from that of its skeleton and soft anatomy, which make up its weight. Nevertheless, the external form has to be understood from the skeleton.
feathers are as important for flight as they are for a warm covering, and the wing and tail feathers, which serve to vary the weight distribution of the soft parts of the body to assist flight, do not follow the soft parts in their modeling as do the smaller down and covering feathers. The wing and tail feathers serve to continue the tensions of the thin muscles and tendons; hence the difficulty in reconciling the shapes of a live and a plucked bird. Only the beak, eyes, and legs are uncovered by feathers.
The usual mistakes made by children and beginners when drawing birds are due to the difficulty of seeing the position of the trunk, spine, and leg bones when they are all covered with feathers. In fact, the human skeleton is closer to the bird's than the quadruped's. The rest position of the bird is closely approximate to that of the human crouching on tiptoe, holding his arms close and bent, so that the hands are at the level of the armpits and hanging down.
A bird's flight position can be imagined by the human spreading out his arms and pushing the edges of the hands backwards; a strong downward pull of the arms corresponds to a beat of the wings. Raising the arms is harder, but a bird is helped by air resistance against the fall of the body. A turn of the arms shows how the bird folds its wings.
The crouching position illustrated shows how the vertebrae, rib cage, and pelvis have become a single, though very elastic, bone structure in the bird. It does not need the flexible spine or shoulder girdle of the human skeleton, for the heavy work of the wings is better served by a firm support, and the strong beat downwards needs to lift the body as directly as possible, so that it is best rigid.
The collar bones have become the solid forked wishbone, which has either grown into a single unit with the breastbone or is joined to it with strong sinew. The shoulder blades have become a narrow saber-shaped bone which often reaches as far back as the pelvis and is thus much restricted in movement. The breast muscles, the motors of flight, weigh as much in birds of flight as all the other muscles together.
They are attached to a strongly protuberant piece of the breastbone, which is not present in birds which do not fly and which is smaller in swimming birds. All muscles other than those of the breast are very thin and model the trunk to an oval or teardrop shape which offers the minimum of Wind resistance. The tail vertebrae do not need much mobility. The last of them have formed a flat plate to which the tail-steering feathers are attached, which, on the whole, point straight backwards. They, like all the feathers, are moved by skin muscles.
The neck vertebrae need greater mobility than in quadrupeds and humans. This is because the eyeballs are almost immobile in their sockets, and because of feeding habits. The number of vertebrae is greater: doves have 12, hens, ducks and birds of prey 13 or 14, geese up to 18, and swans sometimes have 25.
The vertebrae of the neck are surrounded by strong muscles, as those who eat game birds well know. The neck and head are not held with sinews, as in quadrupeds, but with muscles. This is why birds bend their necks in sleep or tuck their heads under their wings, so that the muscles are completely relaxed.