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.