Only two species of cowbirds, Brown-headed and Bronzed, are found in North America. Both cowbird species are generalist parasites, laying their eggs in the nests of a wide range of other species. The Bronzed Cowbird occurs only in the and southwest and extends south into Mexico and through Central America; it has been little studied because it occupies a relatively restricted range in North America and a. somewhat inhospitable habitat. The Brown-headed Cowbird, in contrast, occupies most of our continent south of the Arctic. This reflects the remarkable population explosion and range expansion it has undergone during this century. It has spread from its original home in the Great Plains as humanity has converted forest lands into farms and pastures. In fact, it is now sufficiently numerous to pose a major threat to the continued survival of several species and subspecies that it regularly parasitizes. As a result, much research effort has recently been directed at understanding the breeding biology of Brown-headed Cowbirds, and a surprisingly complex and fascinating picture is emerging.

Although the nests of many species are acceptable places for cowbirds to deposit eggs, all of those species are not necessarily appropriate hosts. Many parasitized species routinely recognize and reject cowbird eggs (by either destroying the egg, rebuilding the nest to cover the egg, or abandoning the nest), while many others are simply inadequate as foster parents and never successfully rear cowbird chicks. Blue-winged Teal, Ferruginous Hawk, Virginia Rail, Killdeer, Spotted Sandpiper, Upland Sandpiper, Wilson's Phalarope, California Gull, Common Tern, Ruby-throated Hummingbird, and Red-headed Woodpecker are among the species that fail as foster parents. The Brown-headed Cowbird now has been recorded as successfully parasitizing 144 of 220 species in whose nests its eggs have been observed. The Bronzed Cowbird has been successful with 28 of 77 species, and of those 28, only 18 occur north of Mexico.

Recent studies estimate that only 3 percent of Brown-headed Cowbird eggs result in adults. In spite of these tremendous losses, the Brown-headed and Bronzed Cowbirds in North America and the Shiny Cowbird in parts of South America and the Caribbean continue to expand their breeding range and numbers. This apparent paradox is explained by the unusual breeding behavior and physiology of Brown-headed Cowbirds (behavior presumably shared by the less-studied Bronzed Cowbirds).

A female Brown-headed Cowbird has a long reproductive period with an extraordinarily short interval between clutches. In fact, this cowbird is the only wild passerine ever reported not to show regression of ovaries and oviducts following clutch completion. Indeed, the physiological demarcation between clutches sometimes is not at all clear, leading ornithologists to characterize female cowbirds as "passerine chickens!" Each female's laying cycle appears adapted to take advantage of a continuous supply of host nests for about a two-month period. An average female lays about 80 eggs, 40 per year for two years. About 3 percent of those 80 eggs end up as adults -- an average of 2.4 adults per female. Clearly, such numbers more than compensate for the excessive loss of eggs and young in the nests of inappropriate hosts. Each pair of cowbirds replaces itself with an average of 1.2 pairs which will double a cowbird population in eight years.

The mating system of Brown-headed Cowbirds shows similar flexibility, ranging from monogamy, to a mixture of monogamy and polygyny, to total promiscuity. The type of mating system seen in a given area is influenced by the spatial distribution of host nests and by the sex ratio (proportions of males and females) of the local cowbird population. Although both sexes occupy distinct breeding home ranges, these areas are not defended and are not exclusive. Where host nests are dense, female home ranges are small, enabling males to guard their mates and resulting in monogamous or polygynous relationships. Where host nests are widely dispersed, female home ranges are rather large, resulting in promiscuous matings as females move over large areas.

A Kirtlands Warbler feeding a young Brown-headed Cowbird. The cowbirds represent a majorthreat to this rare warbler.

Female home ranges are thought to overlap, since eggs of more than one female frequently are found in a single host's nest; a cowbird generally will lay only one egg per nest. Approximately one-third of all parasitized nests hold more than one cowbird egg. A female Brown-headed Cowbird often locates a potential host nest during its construction. She then regularly visits the nest prior to laying while the owners are absent. One day prior to, or on the day she lays her egg, the female cowbird usually removes (and occasionally eats) one host egg from the nest. If only one host egg is present, she does not remove it (otherwise the hosts might abandon their now eggless nest).

Circumstantial evidence indicates that in some areas, at least some female cowbirds specialize in particularly vulnerable host species, to the apparent exclusion of other species nesting nearby that serve as common hosts in other parts of the cowbird's range. A partial explanation could be that host species that have been in contact with cowbirds for a long time have evolved the ability to recognize the parasite or its eggs. As a result, many of these otherwise suitable host species make poor hosts because they aggressively attack female cowbirds, eject or destroy cowbird eggs found in their nests, or abandon their nests altogether upon detection of a parasite's egg. In contrast, many of the most accepting and most heavily affected host species may have been subjected to cowbird parasitism for only a short time because their ranges did not overlap prior to the cowbird's recent range expansion. Examples of species that may have been recently contacted and are now imperiled are Kirtland's Warbler and the California subspecies of Bell's Vireo, the Least Bell's Vireo. There is, however, little evidence as yet to support these "time of contact" explanations.

The only adaptation for parasitism seen in nestling and fledgling cowbirds is their rapid development. Cowbird eggs usually hatch one day ahead of the host's eggs. In addition cowbird nestlings usually are larger and grow faster than the host's young, which enable them to garner more than their fair share of the food brought to the nest. Cowbird fledglings do not recognize their foster parents as individuals, but respond positively to all adults of their foster parents' species. Fledglings receive more food than would the equivalent weight of host young, probably because their loud and persistent calling causes them to be fed more.

Even though some 97 percent of cowbird eggs and nestlings fail to reach adulthood, cowbird parasitism reduces production of young by the parasitized species. Abandonment of a nest by a parasitized host may preclude renesting and result in zero reproduction for that pair that breeding season. The reproductive effort of birds that suffer the presence of a cowbird chick in their nest will be significantly lower than that of unparasitized conspecifics in the same population. Because the cowbirds represent a major threat to many species of passerines, we have paid particular attention to the relative frequency of cowbird parasitism for all documented hosts in the species treatments, and we have designated the frequency with which they are parasitized as "rare," uncommon," "common," and "frequent."

SEE: Brood Parasitism; Conservation of Kirtland's Warbler; Decline of Eastern Songbirds.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.