Page One

On my way to the BART station earlier this summer, I noticed an unfamiliar bird skulking in the shrubbery near the tennis courts at the corner of Martin Luther King and Russell. It was pretty nondescript: bigger than a sparrow, smaller than a robin, pale grayish-brown with vague streaking. But it had this furtive look about it. 

The plain brown bird was, in fact, a female brown-headed cowbird, North America’s best-known avian brood parasite. (There are a couple of other cowbird species with similar proclivities in the Southwest and Florida.) Like the cuckoos of the Old World, cowbirds have dispensed with the onerous routine of building nests and feeding chicks. They simply dump their eggs in another bird’s nest and go on their way. 

Brood parasitism is such an energy-saver that it has evolved independently in several unrelated groups of birds: cuckoos, cowbirds, honeyguides, some African finches, and a South American duck. It’s also been documented in many species of bees and wasps, and even one catfish.  

Fans of Walt Kelly’s immortal Pogo strip may recall the pair of cowbirds who were the sole members of the Okefenokee Swamp’s CPUSA cell, and who tried to infiltrate one of the possum’s presidential campaigns. 

Among the cowbirds, the degree of commitment to the parasitic lifestyle varies. The bay-winged cowbird of South America takes over other birds’ nests, but incubates its own eggs and rears its own young. The screaming cowbird specializes in parasitizing bay-winged cowbirds. Brown-headed cowbirds are generalists, having been known to victimize over 200 other species of birds. They’re not as sophisticated about it as some cuckoos, whose eggs mimic a specific host’s in size and color. But they make up in volume what they lack in deceptiveness. 

Female cowbirds rival battery hens in productivity, averaging 40 eggs a year. If even a small percentage of those eggs hatch and if the chicks are reared to fledging by their foster parents, that’s enough to keep the population going. Cowbirds eggs tend to hatch sooner than those of other birds, and the fast-growing chicks dwarf their nestmates.  

The changelings monopolize the food delivered by the harried parent birds, starving out the hosts’ own young. Lately it’s been discovered that some cowbirds improve their odds by killing any nestlings they find in the host’s nests. (Among cuckoos and honeyguides, it’s the young parasite that does the dirty deed.) 

In their original range—the fringes of the Great Plains, where they followed the bison herds to feed on insects stirred up by all those hooves—brown-headed cowbirds have been at this long enough for their hosts to have evolved counter-measures. Some species toss out the alien egg; others roof over the nest—dooming their own eggs along with the cowbird’s—and start a new brood, or rebuild elsewhere for their second attempt. 

But cowbirds have moved far beyond the Plains, expanding eastward into forest clearings during the colonial era and treating cattle herds as surrogate bison. Their arrival in California was a bit later. They had reached the Colorado River by the late 19th century, then the Salton Sea and the Kern River Valley by about 1910. By 1927, when ornithologists Joseph Grinnell and Margaret Wythe published their “Directory of the Bird-Life of the San Francisco Bay Region,” they had made it as far north as the Bay Area. And they kept on moving, up the coast as far as British Columbia and southeastern Alaska. 

Wherever they went, the cowbirds encountered naïve hosts. Grinnell and Wythe described yellow warblers as commonly nesting in city parks and gardens. No more. Thanks in large part to cowbird parasitism, they’re now rare in the urban Bay Area. Other California birds, like the least Bell’s vireo and southwestern willow flycatcher, have been pushed to the edge of extinction. Bird behavior is pretty much hard-wired, and there just has not been enough time for western species to develop effective defenses. 

There’s another odd thing about cowbirds. In his book “Orioles, Blackbirds, and Their Kin,” Alexander Skutch, who has been observing birds for decades in Costa Rica, describes a bit of behavior called the interspecific preening invitation. Preening—grooming the feathers—is an important social lubricant; it seems to strengthen pair bonds and family ties. But most birds preen only their mates or close relatives. 

Cowbirds, though, will go up to total strangers and solicit preening. And I’m talking about birds of other species, not fellow cowbirds—in one instance, even a caged budgerigar. The other birds appear nonplussed, but most eventually give in. When they don’t, the cowbirds become testy, sometimes physically abusive. 

It’s tempting to relate this somehow to brood parasitism. Skutch speculates that it might function to appease hosts whose nests are about to become cowbird nurseries. But both sexes engage in it, and males seem to have nothing to do with the females’ skullduggery; it’s not a team operation. He suggests that maybe it just feels good to a cowbird to have its head scratched. Who knows? It remains a small mystery, and a salutary reminder that not everything in nature is susceptible to a neat explanation.