[Illustration]
Co-operative nurseries, where a few parents look after the young
while the rest of the adults, temporarily freed of the care of their
offspring, can go about their other affairs, appear in the bird world.
The wild turkey of our Eastern United States commonly steals away
singly to lay its eggs and incubate them in its nest on the ground.
But occasionally it happens, Audubon writes, that several hen turkeys
associate together and lay their eggs in one nest, and raise their
young together. With the turkey apparently there is little division of
labor, as Audubon writes of finding three hens sitting on forty-two
eggs, but he says that one of the hens is always on the watch at the
nest so that natural enemies have no chance to rob it.
A GREGARIOUS BIRD What is of only occasional occurrence in one
species may be the regular course of events in another, and in the
ani we find it customary for a number of birds to nest together. The
anis are moderate-sized cuckoos living in the tropical Americas. The
smooth-billed ani is perhaps the best known, for Dr. D. E. Davis, when
studying at Harvard for his doctor's degree, made a special trip to
Cuba to study them in the field. The smooth-billed ani goes in flocks
the year round. Usually there are about seven birds in the flock, but
there may be as many as twenty-four. The nest is a bulky structure
of twigs and fresh leaves. When nest building starts usually one
bird is most active, but as many as five birds were seen carrying in
sticks at one time. When the nest of sticks and leaves is finished
several females may lay their eggs in it. But apparently only one bird
incubates at a time, and the male takes his turn at incubating. When
the young hatch, after about thirteen days, most of the adults in the
colony help feed the young.
Eider ducks may nest in dense colonies, but each bird has its own nest
in which it lays its own eggs, and in which the female alone incubates.
But after the young hatch and the mother leads them to the water, the
young may band into larger flocks, accompanied by a number of females,
and the young seem to be independent of their particular parent, but
attach themselves to and are tended by the nearest duck.
PENGUIN SOCIAL GROUPS A much more elaborate system for caring for
the young has been evolved by certain penguins. The sexes alternate
in their care of the young in the early stages. But when the young
are partly grown the family unity breaks up for a communistic type
of social organization. The young are now grouped into bands of up
to twenty or more birds and are left under the care of a few old
birds, while the rest of the adults go to the water, which may be
some distance away. Periodically they return with food for the young.
Apparently the individual young is not recognized by the parent, which
goes to the particular group of which its young is a part, and there
may feed any one of the "child groups."
Here we have two definite cases of a social organization that has
resulted in division of labor: in the incubation of the ani, and in the
care of young penguins. In addition we have two less specialized cases
of the same thing, showing the sort of raw material on which evolution
can operate to produce new behavior patterns.