The mother who would leave her infant on a stranger's doorstep, to be brought up an orphan, not even knowing its own parents, is a despicable character in human society. But when we leave the man-made society we must leave man-made rules of behavior and man-made prejudices behind. Morals are human. The rest of the animal world is not immoral, it is amoral. It cannot afford criteria beyond survival and reproduction. So while we call certain birds "social parasites," we attach no stigma to them. They represent several groups: the cowbirds, the weavers, the cuckoos, the honey-guides, and the ducks.
Carelessness in egg laying is common even in birds that ordinarily lay their eggs in their own nest and care for them themselves, as for instance the robins' eggs that you may find on your lawn (which of course are wasted; addling and rotting). Perhaps the fate of the eggs of pheasants and ruffed grouse which are found in the same nest may be more happy. Ducks usually make their own nests, but many species occasionally lay eggs in the nest of another species, and one South American duck no longer makes any nest of its own, but is a social parasite, not only on other kinds of ducks, but also on coots and some other birds.
The small, well-marked family of honey-guides of Africa, notable in other ways, also is remarkable for being social parasites. Their favorite host species, chosen to look after the eggs and young, are their close relatives, the barbets (which themselves are most closely related to our woodpeckers).
The nesting of certain African weaverbirds was long a puzzle to ornithologists until it was found they too were social parasites, on other weaverbirds.
VARIED NESTING HABITS The cowbirds, of several species in North and South America, belong to a family notable for the variation in its nesting habits. Their nests vary from the elaborate purse-shaped structures of the oropendola and orioles to the dome-shaped nest on the ground of the meadow lark, the simple cup of the bobolink and redwing; the cowbird makes none. The cowbird lays its eggs in the nests of a wide variety of other species to be cared for by them. Here those who discuss the relative importance of heredity versus environment can profit by considering these social parasites. The young cowbird, hatched and brought up by, say, a yellow warbler, remains a cowbird. As soon as it no longer needs its foster parents' care it flocks with other cowbirds, with all their mannerisms and characteristics, and next season it mates with another cowbird. There is nothing left of its early environment.
The cuckoos of the New World and some of those of the Old make their own nests in normal avian fashion. But a number of Old World species are social parasites, and their behavior has long been a subject of study and discussion. Specializations indicate that here perhaps we have the highest stages of social parasitism. Whereas the cowbird may grow up with nestmates that are the young of the foster parent, unless perchance it crowds them out or starves them if it is larger, the young cuckoo gets the rightful occupants of the nest on its back and throws them out of the nest to perish.
EGGS LOOK ALIKE Another refinement in social parasitism by the European cuckoo is that apparently certain individuals, and apparently certain strains, lay their eggs only in the nests of certain host species. And these cuckoos' eggs resemble those of the particular species in whose nest the cuckoos' eggs are laid. For example, if certain cuckoos lay their eggs only in the nests of meadow pipits these cuckoos' eggs would resemble those of meadow pipits, while another group of cuckoos specializing in hedge-sparrows would have eggs resembling those of hedge-sparrows. Another oriental cuckoo has a color adaptation in the young. In southern Asia these cuckoos parasitize crows, and the nestling cuckoos have black feathers like the young crows; in the Australian area where the same species of cuckoo occurs it parasitizes grayish-brown honey eaters and the young are brown, more like the rightful nestlings. Both these resemblances apparently reduce the chances of the cuckoos' offspring being rejected by the foster parents.
FISH EATS BIRD! [Ref]
It has become commonplace to hear about birds eating fish. The government gets out reports on the relation of fish-eating birds to fish abundance. The cries of commercial fisheries have caused inquiries to be instituted into the food of cormorants that were supposed to be eating the fish before they grew up enough for us to eat. The scarcity of salmon in some of our Northeastern streams has caused the allocation of biologists to study the predation of kingfisher and merganser on salmon fry and fingerlings.