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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.