Among other things they have learned that while they are feeding their young, birds are especially valuable on a farm. Baby birds require food with a large amount of nourishment in it that can be easily digested. Almost all young birds have soft, tender stomachs, and must be fed on insects; as they grow older, the stomach or gizzard hardens and is capable of grinding hard grain or seeds. The amount of food required by the baby birds is astonishing. At certain stages of their growth they require more than their own weight in insects. And the young birds are to be fed just at the season that insects do the most injury to growing crops of grain and young fruit and vegetables. Birds vary so much in the kind of food eaten, not only by different varieties of the same species, but by the same birds at different seasons, that it is necessary to make a careful study of each bird to know whether, if he is sometimes caught eating cultivated fruit and grains, he helps in other ways enough to pay for it. When insects are unusually abundant, birds eat more than at other times and confine themselves more strictly to an insect diet, so that at such times the good they do is particularly valuable. Birds of prey may do harm in a particular place, because in that region mice, rabbits and other natural food are scarce, and they are driven to feed on things that are useful to man, while in places where their natural food is plentiful the same birds are altogether helpful. In the same way, birds which naturally eat weed seeds frequently find these almost altogether lacking where the farms are most carefully cultivated, but in their place are fields of grain whose seed also furnishes them desirable food. Is it any wonder, then, that, their natural food being taken from them, they turn to the cultivated crops? The fruit eating birds seem always to choose the wild fruits, but where these are not to be had they enter the orchards and soon become known as enemies of the farmer. A careful examination of the harm done by birds leads to the belief that the damage is usually caused by a very large number of one species of birds living in a small area. In such cases so great is the demand for food of a particular kind that the supply is soon exhausted, and the birds turn to the products of the field or orchard. The best conditions exist when there are many varieties of birds in a region, but no one variety in great numbers, for then they eat many kinds of insects and weeds, and do not exhaust all the food supply of one kind. Under such circumstances, too, the insect-eating birds would find plenty of insects without preying on useful products, and the insects would be held in check, so that the damage to crops would be slight. The following are examples of the food eaten by birds and the good that they thus accomplish to man: