Again, as to food, the land birds will be more closely pressed than those which can at all times fish in the sea. There is great scarcity of land food in the winter, while in summer whole flocks of newly-born fledgelings are clamouring for their daily bread. So we shall find that every kind of eatable thing is turned to account, and we have among land birds seed-eaters, vegetable-feeders, and fruit-eaters; insect-devourers, and feeders on slugs and worms and snails; and flesh-eaters which feed on other birds, or on mice, bats, and larger animals; while large flocks of birds of all kinds visit different parts of the earth in the various seasons, going north in summer to build their nests, and south in winter, in search of food. All these birds live chiefly in the air; while on the ground there are the scratchers--fowls, partridges, turkeys, and grouse, which rake out the hidden grains, and rarely rise into the air except when they are frightened, or to roost on the trees at night. And between these ground birds and the true tree birds we have the doves and pigeons, some of which feed on fallen seed and grains, and others on fruit. And each and all of these birds have some difference in beak and claw, in their manner of nest building and rearing their young, and in their habits and ways, which enables them to make the most of their lives. * * * * * Even nest building does not come to all land birds by nature, and, as we shall see, it depends very largely on the habits and the structure of the builders. Thus the Partridges, and their relations the Pheasants and Grouse, lay their eggs in the thick grass of the meadows or among the heather, and at most sometimes scratch together a few dry grass blades for a bed. In this they remind us much of the ostrich family, which also scrape a hole in the ground for their eggs and scratch food for their children; and in fact there is a group of curious heavily-flying birds, called Tinamous, in South America, which are so like quails and partridges on the one hand, and ostriches on the other, that they lead us to wonder whether it was not from the ancestors of such birds as these in ancient times that the heavy running birds started on one road, while the lighter and swifter birds took to the wing.