As you have watched the birds in their flight, I daresay you have wondered how they can keep themselves up in the air. Even the little wren has some weight; much more the crows which make their nests in the topmost branches of the trees. We say "as light as a feather"; yet the downiest feather has some weight, and will find its way to the ground if not kept up by wind or breath.
It is true that the "feathered fowl," as all kinds of birds are called in the Bible, are very much heavier than the air in which they float and swim, using their wings for oars, just as the fish use their fins. But do you remember that little balloon inside the fish, which enables it to rise through the water? A bird is almost a live balloon; as it flies, it breathes air into every part of its body; this air becomes heated, and is kept warm by the feathers; and as hot air becomes light, the bird is so much lighter than the air which surrounds it, that it can easily rise higher and higher, until, like the skylark, its little quivering body seems almost lost in the far blue sky, and its "waterfall of song" alone shows where it is.
[Illustration: "THE WHITE SEA-GULL, THE BOLD SEA-GULL, A JOYFUL BIRD IS
HE.">[
The bones of a bird are very strong, but they are also very light; if you look at the bones of a chicken, you will see that some of them are hollow; when the bird was alive, those hollow places were all filled with air. Take a dead bird and look at the quills at the roots of the feathers; and now watch that swallow as it darts so rapidly hither and thither. The bird is able to fill each tiny quill with air, so that its body becomes like a balloon, and it rises high above the roofs of the houses; then, like the fish, when it wishes to sink, it can breathe out all the air again, and so constantly change its weight, and fly, now high, now low, faster than any train can rush or ship sail.
There is a wonderful bird which sailors have seen a thousand miles from land. It is called the Frigate-bird, and has never been known to rest on the sea; it lives upon sea-creatures, but makes its nest on shore. Each of its wings, if stretched out as when the bird is flying, measures more than the height of a man; yet even such an enormous bird as this does not sink down by its own weight, but flies mile after mile upon its strong wings, every feather of which unites strength and lightness, never resting till its airy voyage is over, and it finds its nest. It is said that when storms sweep over the sea, this "ocean eagle" mounts upward until it has reached the calm which lies above the storm, and so sails upon its untroubled way.
The feathers of birds are to them what its scales are to the fish, and hair and wool to other animals—a protection. They are not only light and strong, but warm, and by their means, as a bird soars into colder regions of air, it is protected from the cold: while for aquatic birds there is a special provision—by pressing with their beaks an oil-gland near the tail they can waterproof their feathers! Now look again at your dead bird; you will see that the wings and tail are formed of quills, while the surface of the body is covered with short feathers—even the ear being protected by a little tuft—and all the spaces between are filled with the softest, warmest down. Could any creature be more beautifully equipped for its journey through the fields of air?
Then this soft, warm, light dress is renewed once or twice a year, generally so gradually that the change is imperceptible—but you may have seen fowls and ducks straggling about the farmyard with half their feathers gone—on the principle of being off with the old coat before they are on with the new.
The eyes of both fishes and birds have an extra lid formed of very thin skin, which can be moved quickly over the surface of the eye, serving to cleanse it and protect it.
There are three thousand distinct kinds of birds, but it would be impossible to learn about so many, they have been divided into five groups—birds of Prey, Perching birds, Scratching birds, Wading birds, and Swimming birds.
I must tell you that Chrissie and Sharley and May had learnt something about these groups from a book of which they are very fond; it is called The First Year of Scientific Knowledge, and there are pictures in it of the different birds, beasts, and fishes which are mentioned.