So the deserts and plains have their ostriches and cassowaries, the open ocean its albatrosses and its penguins, the shores their ducks, gulls, and waders, and the little inland pools and marshes their water-birds, which come there to build their nests and seek for food. Yet these are after all not by any means the larger portion of the bird world. It is in the woods and forests, the moors and pastures, on the solitary mountain peaks above, and in the snug valleys nestling below, that we find the myriads of song birds and game birds and birds of prey; of climbing birds such as the Woodpeckers; swiftly sailing birds such as the Swifts, cooing Wood-pigeons and cawing Rooks; terrible Eagles and Hawks, or sweet-singing Nightingales and Thrushes. All these birds have had a very different education from that of the far-sailing albatross or the running ostrich. They have grown up in the midst of innumerable dangers; for enemies of all kinds--beasts and reptiles and other birds--live all round about them, making food scarce and destroying their young, so that of the millions born into the world thousands upon thousands perish every year before they grow up. We should expect, then, that these land birds would learn many devices for protecting themselves and their little ones. The guillemot can afford to lay her egg on the bare rock, for few animals can climb the high cliffs where she makes her home; and the penguin on her solitary island may lay hers in the mud on the ground. But the little lark must look carefully for high grass in which to build her nest, and the rook must weave a strong basket-work of twigs to make a home for her nestlings in the top of the high elm. Moreover, the land birds cannot sleep safely on the ground, where weasels and stoats, foxes and wild cats, prowl by night in search of prey; they must take their rest on the boughs of the tall trees and cling on by their toes even when they are in the deepest slumber. This they could not do if they had the stumpy cushioned feet of the ostrich, the webbed feet of the duck, or the flat three-toed feet of the waders. It is the fourth toe turned backwards, and growing very long in many of the perching birds, which gives them their grasp; while a special muscle, beginning behind the thigh (_th_, Fig. 33, p. 126), coming round over the front of the knee (_k_), and then passing behind the heel (_h_), and on to the toes, keeps them bent. Picture for a moment this muscle sending its cords or tendons from behind the leg over the knee, and then drawn back by the heel, and you will see that the more heavily the bird sleeps, pressing upon its legs, the more the knees will be bent forward, the tighter the cord must be stretched, and the stronger the grasp will be upon the bough.