Brood Parasitism

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.