Some
species of birds thrive not by carefully rearing their own
young, but by pawning that task off on adults of other
species. The European Cuckoo, whose distinctive call is
immortalized in the sound of the "cuckoo clock," is the bird
in which this habit has been most thoroughly studied. Female
European Cuckoos lay their eggs only in the nests of other
species of birds. A cuckoo egg usually closely mimics the
eggs of the host (one of whose eggs is often removed by the
cuckoo). The host may recognize the intruding egg and
abandon the nest, or it may incubate and hatch the cuckoo
egg. Shortly after hatching, the young European Cuckoo,
using a scoop-like depression on its back, instinctively
shoves over the edge of the nest any solid object that it
contacts. With the disappearance of their eggs and rightful
young, the foster parents are free to devote all of their
care to the young cuckoo. Frequently this is an awesome
task, since the cuckoo chick often grows much larger than
the host adults long before it can care for itself. One of
the tragicomic scenes in nature is a pair of small foster
parents working like Sisyphus to keep up with the voracious
appetite of an outsized young cuckoo. Interestingly, different
females within a population of European Cuckoos often
parasitize different host species. Some cuckoos may
specialize in parasitizing the nests of Garden Warblers;
others of the same population may lay in the nests of Reed
Warblers, and yet others may lay in nests of White Wagtails.
The eggs of each female very closely mimic those of the host
selected (even though one host may have large, densely
spotted eggs, and another may have smaller, unmarked pale
blue eggs), and the mimetic patterns are genetically
determined. The different genetic kinds of females (called
"gentes") apparently mate at random with males. How these
gentes are maintained within the cuckoo populations is not
fully understood. The North American
Yellow-billed and Black-billed Cuckoos only rarely lay their
eggs in the nests of other species, but occasionally lay
some of their eggs in the nests of other members of their
species. Our cuckoos usually build nests of their own and
rear their own young. Only about 40 percent of cuckoo
species worldwide are brood parasites, the rest care for
their own eggs and young. Brood parasitism is much
less common in other groups of birds. It is found in about 1
percent of bird species, including members of such diverse
groups as ducks, weavers, and cowbirds. In North America the
only obligate brood parasites (those which must parasitize
and cannot build nests of their own) are the Bronzed and
Brown-headed Cowbirds, which may be important enemies of
other birds. The Brown-headed Cowbird has been recorded as a
parasite of more than 200 other species. Cowbird eggs do not
closely mimic host eggs, nor do the young oust host eggs and
young from the nest. But cowbirds do tend to hatch earlier
than their hosts, to grow faster, and to crowd out or at
least to reduce the food intake of the host's
young. Cowbirds thus can place
powerful selection pressure on a host bird species to learn
to recognize and reject cowbird eggs. Behaviorist Stephen
Rothstein of the University of California at Santa Barbara
has shown experimentally that some North American species
have, indeed, learned to do this. He placed artificial and
real Brown-headed Cowbird eggs in the nests of 43 other
species, and found that those species divided rather neatly
into acceptor species and "rejector species." Acceptors
include many warblers, vireos, phoebes, and Song Sparrows,
while robins, catbirds, Blue jays, and Brown Thrashers are
rejectors. The Song Sparrow just happens to have eggs very
similar in size and spotting pattern to those of the
cowbird, and almost invariably raises the cowbird young. In
contrast, catbirds and robins, which lay unmarked blue eggs,
almost invariably eject cowbird eggs from their nests.
Phoebes, strangely, usually have unmarked eggs but are
acceptors -- perhaps their habit of nesting in dark recesses
has reduced their awareness of egg pattern. Rothstein found very little
sign of transitional species -- that is, ones with some
individuals that accepted and others that rejected. The
reason, he hypothesized, was that once the genetic ability
to reject appeared in a species, it would spread very
rapidly and very soon all individuals would be rejectors.
That notion is certainly supported by rates of parasitism
observed in acceptor species. In various studies, for
example, 40 to 70 percent of the nests of Red-eyed Vireos
were parasitized, about 20 percent of Eastern Phoebe nests
were parasitized, and about 40 percent of Song Sparrow nests
were parasitized. Reduction in the fledging rate of
parasitized nests was well over 50 percent in the vireos and
phoebes, and about 40 percent in the Song
Sparrows. A central mystery remains,
however. Acceptors and rejectors do not represent different
taxonomic groups, they do not have different lengths of
association with cowbirds, most have eggs that can be easily
distinguished from cowbird eggs, and they are capable of
ejecting cowbird eggs from their nests. Why then have some
species evolved the ability to reject, and others not.
Rothstein suggests that nest concealment, large bill (to
make ejection easier), and chance may play key roles. More
careful observation and experiment will be needed before we
will know if he is right. Brood parasitism is not
restricted to females of one species laying eggs in the
nests of other species. In addition to some of our North
American cuckoos, females of a wide variety of species
sometimes lay eggs in the nests of other females of the same
species. This behavior is examined in other
essays. SEE: Parasitized
Ducks;
Parasitic
Swallows;
Coevolution;
The
Decline of Eastern
Songbirds;
Monogamy;
Conservation
of Kirtland's Warbler;
Cowbirds. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.