In most birds the young remain with the parents for a time after leaving the nest, and gradually become self-reliant, so that they procure their own food. At this stage family parties usually separate as the young drift away from their parents, or the adults tire of importunate begging for food on the part of progeny able to procure their own sustenance, and so either drive the young away or themselves leave. When the young learn early to search for food they may remain in bands with their parents.
Parasitism.—Breeding parasitism is found in several groups of birds in which no nest is built, the eggs being deposited with those of other birds and left entirely to the foster-parent for care. The European cuckoo (Cuculus canorus) has long been known to have such habits. The female places a single egg in the nest of some bird, usually a species of small size, ordinarily choosing a nest in which the eggs are fresh, and removing one of the rightful set when leaving her own (see CUCKOO.) Although about i oo species of birds have been recorded as parasitized, ordinarily each female cuckoo places her eggs in the nests of one particular species of bird. Each female cuckoo is supposed to lay from five to 16 eggs during the season. There is argument as to whether the cuckoo deposits her egg directly in the nest of the fosterer or whether she extrudes the egg elsewhere and carries it in her throat to the nest chosen. The former is certainly the more usual. An egg of the f oster parent is almost invariably removed and eaten by the cuckoo after she has deposited her own. The process of placing the egg requires only a few seconds, and then the bird leaves at once, which renders • definite observation difficult. The young cuckoo's back is hyper sensitive to touch, has a spasmodic reaction when it comes in con tact with objects other than the nest ; aided by a curious depres sion on the back, the parasite casts out the rightful young, or the eggs until finally it occupies the nest alone.
Similar parasitic habits are known for a number of other cuckoos in other parts of the world. Stuart Baker, in discussing parasitism in Indian cuckoos, shows that a number are parasitic on one species or group of species of birds, and that the cuckoo's egg is specialized to resemble the foster-parent's rather closely. " In some cases one cuckoo may have widely different eggs in different parts of its range, as the hawk-cuckoo (Hierococcyx sparveroides), which, when it parasitizes the streaked spider-hunter (Arachnothera magna), lays a dark olive-brown egg like that of the bird para sitized, but that elsewhere foists on the laughing thrushes or allied species a bright blue egg like that of its dupes.
Some of the American cowbirds are also parasitic. The common North American cowbird (Molothrus ater) has developed no par ticular resemblance in egg colour to the egg of the fosterer, except that many of its dupes lay spotted eggs like its own. In South America the bay-winged cowbird (Molothrus badius) cares for its own eggs in a normal manner, but is parasitized by a related species, the screaming cowbird (Molothrus rufo-axillaris), which uses this species alone as foster parent. Parasitism is also known in the rice-grackles (Cassidix orizivora) which lay in the nests of related oriole-like birds. Certain species of weaver-birds (Ploceidae) and the honey-guides (Indicatoridae) of Africa also parasitize other birds. The origin of such habits is obscure.
The period of reproduction is the most active part of the annual life-cycle of the bird, from a biological standpoint, as it brings in play-instincts and activities different and more complex than those apparent during the remainder of the year. Reproductive activity reaches its climax with the maturity of the young. With these on the wing the individual pair have done their utmost in the perpetuation of their kind, and there ensue a number of months of more restful life, with search for food and escape from enemies as the principal activities. For many species there is immediately a period of moult to renew the feathers, a period during which birds are quiet and sluggish, often seeking haunts where they will be disturbed as little as possible. With the drain on vitality brought by feather-production at an end, there comes a time of greater activity, when old and young seem to attain greater vitality.