The next order, that of the perching birds, includes all our common song-birds, such as the robin, bluebird, and blackbird, as well as a few larger birds, like the crow.
The scratching birds form another order, including our domestic fowls and many wild game-birds.
The next order comprises the ostrich and a few other large birds, which have such small wings that they are unable to fly, but with very large and powerful legs, so that they are excellent runners. Although this order includes the largest bird at present living, there were formerly running birds very much larger than any which now exist; for, in Madagascar and New Zealand, the bones, and even the eggs, of gigantic birds have been found. One of these eggs was over a foot in length, and contained more than ten quarts or as much as six ostrich eggs or one hundred and fifty hen's eggs. A nearly complete skeleton of one of these birds has been found, and this must have belonged to a bird fifteen feet high, or taller than the largest elephant!
The next order includes the wading birds such as the snipe, plover, woodcock, heron, and rail.
Another order is that of the gulls, ducks, geese, pelicans, penguins, and other swimming birds.
Besides these living birds, fossil birds have been found in the rocks. Some of these are very different from any species now living, and very much like reptiles, so that it is not easy to decide whether they are to be called birds or reptiles.
The chief peculiarity of birds is their power of flight, and, although there are a few birds which do not fly, most of them do, and the various organs of their bodies are all constructed in such a way as to fit them for a life in the air. Their bodies are very solid and compact, in order that most of their weight shall be near the place where the wings are attached. The feet, legs, head, and neck are light, and so arranged that they may be drawn up close to the body while the bird is flying. As the neck is long and very flexible, the body does not need to be pliant, as with most creatures having backbones; but it is important that the wings should have a firm support, so the bones of the back are united. The body of a bird must also be well protected from the cold; for, as it ascends and descends through the air, it passes through regions of very different temperatures, and it must be provided with a thick and warm covering in order to be able to endure these sudden changes, and one also which shall be very light and able to shed the water; for, otherwise, a bird would be unable to fly. The feathers of a bird answer to all these needs, and are so placed upon the body that they form a smooth surface which does not catch against the air when the bird is passing through it. In its rapid ascents and descents, the bird is exposed to another danger even greater than the sudden changes of temperature. You all know that air presses in every direction with great force, and that we do not feel it because there is air in all parts of our bodies as well as outside them, and the pressure of the air inside exactly balances that of the outside air. If we should suddenly take away the outside air in any way, such as covering a person up with an air-pump receiver, and quickly and completely exhausting the air, the consequences of the inside pressure would be very terrible, and if the experiment could be tried quickly enough the body would burst like an exploding gun, with a loud noise.
THE EAGLE (BIRD OF PREY).
When people go up rapidly in a balloon or climb very high mountains, they are troubled by a ringing noise and a feeling of great pressure in the ears and head, and by palpitation of the heart, bleeding at the nose, and fainting. These unpleasant and often dangerous symptoms are caused by the expansion of the air inside their bodies. In ascending very high mountains it is necessary to go very slowly and to stop very often, to give time for some of the expanded air to escape, and equalize the pressure again. Now, many birds, the condor, for example, fly over the tops of the highest mountains, and nearly all birds, either occasionally or habitually, ascend to very great altitudes, and, unless there were some plan for regulating the pressure of the air inside their bodies, they would suffer great inconvenience and even pain and danger. But they are provided with an arrangement by which the air within them can escape easily as it expands and thus keep the pressure within just equal to that outside, so that they can ascend and descend as rapidly as they wish, without feeling the least inconvenience. In the body of the bird there are several large bags, like the lungs, called air-chambers; many of their bones are hollow, and others are pierced with long winding tubes called air-tubes. All these air-chambers and air-tubes are connected with the lungs so that air can pass into and out of them at each breath. The connection between these chambers and the lungs is so complete that a wounded hawk can breathe through a broken wing almost as well as through its mouth. When a bird mounts upward, the air inside its body gradually expands, but the bird does not feel any inconvenience; for, at each breath, part of the air passes from the air-chambers into the lungs, so that the pressure on the inside does not become greater than that on the outside.