A few weeks ago a York neighbor called with news she had adopted a baby bird, one she was sure had been abandoned by its parents. From her descriptions of the parent birds--one red and one yellow--we guessed she had rescued a young Summer Tanager.

Summer Tanagers are relatively common birds that breed here in the Piedmont and overwinter in Central America. The male's plumage is similar to that of the Northern Cardinal, but the mustard-colored feathers of the female make it hard to confuse her with anything else. According to my neighbor, the adult tanagers fledged four young, three of which could fly, but one of which was a "runt" that got left behind by more vigorous nest mates.

After several days of hand-rearing, the adoptee (nicknamed "Bee-Bop") was strong and well-feathered enough to be released, so the neighbor brought him to Hilton Pond for me to band. The first question she asked was: "When will this fledgling exchange its drab gray-brown plumage for typical tanager colors?" She was startled when I told her it would never happen.

As soon as I saw "Bee-Bop" I realized my neighbor was not the first foster parent of this plain little bird--I knew the adult tanagers had played that role before she did. What both my neighbor and the tanagers had adopted was a young Brown-headed Cowbird, social parasite of the American bird community.

Brown-headed Cowbirds--members of the blackbird family--are named for the mahogany-colored head feathers of the male, who is otherwise glossy black (top photo). Adult females (below) bear the nondescript pale brown coloration sported by juveniles such as "Bee-Bop," who I could positively identify as to species and sex because his wings were molting to black and his drab natal head feathers were changing to a rich brown.

One of the most amazing things about "Bee-Bop" is that his species is probably not native to the Piedmont or even to South Carolina, and how his early relatives got here is an interesting story.

Cowbirds might be more aptly named "buffalo-birds," since they originated in the open prairies and fed on insects stirred up by bison. Today the species occurs in virtually every state east of the Rockies, a feat made possible only because of interference by humans. When early settlers cleared the great eastern forests, they produced fields--artificial prairies of sorts--that attracted cowbirds and provided corridors for their dispersal.

Like other prairie birds, cowbirds originally nested in the only sites available--the grasses and short shrubs that blanketed the Great Plains. (There’s some disagreement about whether cowbirds historically occurred in the Piedmont prairies common in pre-settlement days in the Charlotte NC region.) Even though they built their own nests, cowbirds occasionally may have laid eggs in the nests of unrelated species; such behavior is noted rarely in other kinds of birds. This egg-dropping provided an efficient way to produce more offspring than usual: While cowbird parents were caring for cowbird babies, the foster parents were doing the same with a few "extra" baby cowbirds. As this penchant for plopping eggs in other species' nests became more and more common over evolutionary time, Brown-headed Cowbirds (and their close relatives, the Bronzed Cowbirds) became the only North American birds to "forget" how to build their own nests.

When cowbirds became totally dependent on other species to raise their young, they turned into what ecologists call "obligate social parasites." (Some ducks and the American cuckoos are "non-obligate" parasites who sometimes deposit eggs in nests other than their own.) Relieved from the usual burdens of nest-building, incubation, brooding, and caring for fledglings, cowbirds ended up with a lot more "free time," and they adapted by laying more eggs than might be expected from a typical songbird. Female cowbirds lay an egg a day for about five consecutive days, take a short break, and repeat the cycle twice more for a seasonal total of 15 or more eggs, all laid in different nests. This helps explain the population explosion experienced by cowbirds since the turn of the century.

Historically, cowbirds parasitized the relatively few species of birds that nested on the open plains. Now that they have invaded the eastern U.S. with its hodge-podge of forests, shrublands, and open fields, cowbirds have many more types of nests to choose from, and their eggs have been found along with those of at least 210 species of North American birds.

Some parasitized species will not tolerate a cowbird egg in the clutch, and Blue Jays and Gray Catbirds are known to throw them out. Other species simply abandon a parasitized nest, while Yellow Warblers may build a whole new nest on top of their first one, covering the cowbird egg as well as their own. Most foster parents, however, appear unable to recognize a misfit in the clutch, and when a cowbird baby hatches, it gets fed with the same dedication as the true offspring. Actually, the aggressive cowbird chick may get the most food, and its faster growth virtually assures its survival, sometimes with the loss of a nestmate or two. Cowbird babies are often twice or thrice the size of their foster parents, and it taxes the adults to bring enough food to satisfy their own offspring, much less an oversized adoptee.

Vireos, wood warblers, towhees, and sparrows appear to be "favorite" hosts for the parasitic cowbird. Some ornithologists suspect that these species suffer greatly, especially since a cowbird typically removes one of the host bird's eggs as she lays her own. In the case of Kirtland's Warbler, a rare and endangered species that breeds in Michigan's jackpine forests, researchers have found it necessary to trap adult cowbirds and also remove their eggs, lest they be the final straw that brings about the warbler's extinction.

There is no doubt that Brown-headed Cowbirds are increasing in the Piedmont. One of the first known parasitizations in South Carolina occurred in 1978 when my students and I found a cowbird egg at Mac Stewart's farm east of York. We were drift-floating an old pond in search of snakes that sun in overhanging shrubs when we spooked a female Blue Grosbeak from the foliage. We climbed off the floats and found a nest containing two of the grosbeak's bright blue eggs--plus a tan one with brown speckles typical of the cowbird.

We checked this nest daily for the next three weeks, documenting the incubation period, the hatch date, and the rapid development of the cowbird chick, which apparently fledged along with both grosbeak babies. Since that time, other field workers have reported additional cowbird eggs, and this year I found one in another Blue Grosbeak nest on my property at Hilton Pond.

I took several photographs of the new nest and banded the four chicks. The adult grosbeaks nested once more after fledging their mixed brood, but there are no signs of cowbird eggs in the second clutch. Maybe the way to beat cowbirds is to wait until later in the breeding season to lay the first set of eggs.

There's no doubt that Brown-headed Cowbirds are destined to be permanent avian fixtures here in the Piedmont. It remains to be seen whether local birds can adapt to and rebound from the pressures of this resourceful invader. Our "native" species have had cowbirds as nest parasites for only a few years, and local birds may thwart them eventually by nesting later or more secretively, or by recognizing and destroying cowbird eggs.

It's worth recollecting that this whole scenario developed, as often happens, when humans tampered with an ecosystem. By cutting down large tracts of trees and establishing open areas, settlers tilted a delicate natural balance in favor of cowbirds. Such activities often have far-ranging effects, and it may take several generations of Piedmont naturalists to understand the ramifications of allowing Brown-headed Cowbirds to invade eastern North America.