The skull of the bird is unlike that of the quadruped or human. It consists mainly of eyes and beak.
If a man had eyes of the same proportionate size as those of an eagle, they would be as large as tangerines; and of the great horned owl, even larger.
Bird's legs differ in the same way from human's as hoofed animals; only the toes remain. The very small thighbone when at rest is almost at right angles to the lower thighbone, and the shinbone is almost nonexistent. The instep has become a single bone, the bird's "leg."
Generally, birds have four toes, one of which points backwards. Climbing birds, like parrots, have two toes pointing backwards; in water birds they all point forwards. Many birds have only two toes; the African ostrich has three.
The number of knuckles also varies. The toe pointing backwards has two, the inner, forward toe three, the second four, and the outer toe five. Whereas all the leg joints function like hinge joints, the two outer, forward toes have rotary joints, enabling them to be pulled towards the inner one.
The hip joint is still a ball joint, but it is so stiffened with tendons that an inward movement of the leg is possible to bring the leg under the line of grpvity of the body, and in stepping forward the body can be moved in a straight line. More broadly built water birds cannot do this; they have to waddle, with inturned toes.
The "arm" bones of the bird have been completely adapted for flight, but they still resemble human arms very closely, and even human hands. The humerus and ulna, however, have grown together at the ends and cannot rotate around each other but form a strong plate to which the arm pinions are attached.
The wrist and finger bones have dwindled or disappeared completely, except for two extended bones of the first finger, which hold the hand pinions attached by a hinge joint which prevents the wing from bending too far either up or down.
The feathers present the draftsman and painter with a difficult problem if he does not wish to copy every detail naively and exactly. Basically, the feathers are a mass which, like a tiled roof, could be done with a texture; but this does not work, for the texture has. to cover a relatively complex underlying form, and the difference of the feather's shape and its function must be made clear in places. There are two main types of feathers: the small, soft down feathers, which are indefinite in shape, and fhe covering, tail, and wing feathers, which are harder and clearer in shape. Down feathers, which provide warmth, correspond to the coat of quadrupeds. The covering feathers lie over the down and are more of a mechanical protection.
Their size and definite shape make a clearly marked scaling which has to be reproduced lying correctly.
Every feather is composed of a quill and the feathering, which is made up of numerous small branches standing out flat to left and right of the upper part of the quill-symmetrically in the tail, covering, and down feathers, and asymmetrically in the wing pinions. The feathering of down is soft and curly, but on the others it is harder and cleverly toothed to form airproof plates. The narrower feathering of the wing pinions always points away from the body and overlaps like roof tiles, looking at the bird's back as a roof ridge. When the wing beats down, this layering causes the air to press the feathers together, and when The wing is lifted, they are opened, like the slats of a Venetian blind.
The. wing and tail feathers serve to increase considerably the area of the bird's body when it is flying, without adding much to its weight. The shape of tire bird when flying cannot be deduced from its bodily structure, but must be studied carefully in each case. Almost every bird has its characteristic flying shape.
The feathers are stuck at an angle into the skin at the lower end of the round quill and, like beak and claws, are extensions of the skin. They are moved by numerous small skin muscles, which can ruffle them and guide the movements of tail and wing feathers for steering.
Lastly, we should mention the decorative growth of feathers, crests, and unusually long tail feathers. They serve no practical purpose, but rather tend to restrict freedom of movement and flight. Other decorative skin forms are the strange combs and throat flaps on some birds.
Birds in all their variety of species have been used more than any other animals for ornamental stylization, especially emblems. The most interesting examples of naturalistic representation of birds are those of the ancient Egyptians, who solved the problem of reproducing feathers by drawing the most important tail and wing quills very exactly and leaving the others out, giving only the modeling or contour, but again putting in carefully observed details of eye, beak, and gait.
Egyptian and Chinese wall paintings are full of fascinating, closely observed pictures of flight. In the Baroque and Rococo periods artists began to take an interest in the beauty of different species of birds and made delightful colored copper plate pictures of them, which are still the source of many lovely bird books. Might it not be a stimulating change from the eternal still lifes and bathing nudes to work on pictures of birds?