The ribs of the bird form a large, light, boat-shaped frame. Its tail is made of a number of backbone pieces, crowded close together. This tail moves easily, and serves the bird as a rudder, to steer and steady its way through the air. If you strip all the meat off a chicken’s wing-bones, you will see a kind of a rude pattern of a man’s arm and hand, with a thumb and two fingers. The length of the wing usually depends on the length of the upper wing-bone. But in humming-birds and swifts, the lower part of the wing is the longer. In some birds, the wing, when spread out, is much longer than all the body. The wing folds up, something like a crab’s claw, or as you would shut a knife. In birds that are walkers, and not flyers, the first wing-bone is short. The penguin, which never flies, has a little short wing, which looks like a hand, without any arm. Now, we come to the bird’s legs. In your leg, the thigh-bone, the one from the hip to the knee, is the longest. In birds, it is the shortest. Even in very long-legged birds, as the stork, the thigh-bone is short. The length comes in the bone from the knee to the foot. Thus, when the bird flies, its large, hollow, air-filled thigh-bone is drawn up to the body, and the lower part of the leg is held backward, and does not hinder the flight. The foot of most birds has four toes. But some, as the ostrich, have less. I shall talk to you about feet in the next lesson. The first thing you notice about the bird’s head is the beak. I shall tell you about beaks, also, in the next lesson. Now, I shall only say, that the beak is the horny, outer end of the jaw-bone. Birds never have any teeth. The beak often forms the larger part of the bird’s head. The head is small for the body, and nearly flat on top. When you see a bird with two things like horns on its head, as some owls have, do not think that they are either horns or ears. Birds have no horns. These things are only feather head-dresses. Birds have ears, but the ear-hole is hidden under the feathers. Birds have very keen sight, very keen power of smell.[24] They have loud, clear voices. Many of them have very sweet voices. I suppose they have as good a sense of taste as we have. I notice that parrots like sugar and cake, and many birds like fruit. What do birds have instead of teeth? They have inside of the body a gizzard. It is part of the stomach. It is so made that it can grind up the food as teeth would. Some birds swallow bits of shell and gravel, which help the grinding. The food goes into a bag called a crop. From the crop it goes to the gizzard, where it is ground. Mother birds feed their very young birds with food which has been softened in this way.