=Social relations of birds.= In aërial birds (except the cuckoos) the male and female pair, build a nest, and both take part in the incubation of the eggs and the feeding of the young. Usually a pair once mated remain mated for life and are very devoted to each other. In wild land birds the pairing habit is not of advantage to a species, but still the tendency to single matings is very strong. When land and water birds are domesticated man tries to break them of this habit because the males produce no eggs and he prefers to eat them while they are young and their flesh is tender. But, as will appear in detail when the different species of birds of this class are described, he does not always succeed in doing this. Even the domestic fowl and duck, in which pairing has been prevented for centuries, often show a strong tendency to pair; and the females with broods of young usually separate from the flock until the little ones no longer need their care. With this separate family life there is still in most species of birds concerted action by communities in migrations, in forming colonies, in attacks on other creatures, and in defense from enemies. From the earliest times of which we have knowledge the devotion of birds to their mates and to their young has afforded the most common and most beautiful illustration of family life in nature. =Place of birds in domestication.= The place of birds among domestic animals corresponds to that of garden vegetables, small fruits, and flowers among cultivated plants. The great staple agricultural crops--corn, wheat, oats, barley, hay, apples, oranges, horses, cattle, sheep, hogs, etc.--are produced mostly by men who make farming and stock-growing their business. But, while large quantities of garden vegetables, small fruits, flowers, poultry, pigeons, etc. are grown by people who specialize in them, the greater part of the supply in all lands comes from small gardens and small flocks on ordinary farms and in the back yards of town homes. =Uses of birds in domestication.= With the exception of the cage birds and the ostrich, all our domestic birds are valuable for their flesh; but, as some kinds can be produced more easily and cheaply than others, people growing birds for the table give most attention to those that can be grown most profitably, and the others are grown principally by those who prize them for rarity, beauty, or some peculiar quality. The eggs of all birds are edible, but birds differ greatly in the number of eggs that they lay and in the disposition to lay them in places provided for the purpose. So, nearly all who keep birds for their eggs keep fowls, which are the most prolific and most docile, and hens' eggs are the staple eggs in the markets.