Some folks can’t stand house sparrows (a native of north Africa and Eurasia) while others detest starlings (a native of Europe). Both species were introduced into this country in the 19th century. While I don’t especially admire house sparrows and starlings, my favorite bird to despise is the brown-headed cowbird, a native of North America.
The brown-headed cowbird is the black sheep of the blackbird family, which numbers among its kind such upright and attractive denizens of the bird world as bobolinks, meadowlarks, red-winged blackbirds, and Baltimore orioles. Unlike most blackbirds, which have long sharp-pointed bills, the cowbird displays a short sparrow-like bill.The male’s lower body has the shiny-black coloration of, say, a grackle, but its head is glossy-brown. The female is a plain gray-brown above, paler below.
I often hear cowbirds before seeing them. They “sing” a squeaky, not entirely unpleasant, “glug-glug-glee” gurgling song and emit a call that is a sort of rattling “check.” Look up and you’ll spot them perched on an extended branch or wire. They seem to teeter back and forth on their perches like drunken high wire artists. But, alas, they never fall.
Here comes the bad part. Along with its cousins the shiny cowbirds, a South American species that appears in the Deep South, and the western bronzed cowbirds, the brown-headed cowbird is the only North American songbird that regularly practices “brood parasitism,” which is a fancy way of saying that it lays its eggs in the nests of other birds and leaves the rearing of its young up to them. Yellow-billed and black-billed cuckoos will rarely lay an egg in another species’ nest.
According to Fred Alsop’s Birds of the Smokies (1991), the cowbird’s “scientific name (Molothus ater) can be translated to ‘black parasite,’ [as] the female selects the active nest of another species ... and lays her eggs there, often removing an egg of the host for each one she lays .... Fledgling cowbirds seem to be perpetually famished and my attention has often been drawn to the sight of a scurrying vireo or song sparrow feverishly trying to collect and transport insect after insect to the gaping mouth of its constantly calling ‘baby’ cowbird. The foster child is often considerably larger than the attendant ‘parent.’”
The brown-headed cowbird will lay their eggs in the nests of over 75 other species, mostly those smaller than themselves. Each female deposits up to 25 or more eggs per nesting season. The energy toll this takes on the hosts, which can’t seem to resist the urge to raise the ravenous baby cowbirds, is enormous.
It’s estimated that well over a million cowbird eggs are laid every year. Not a single one is laid in a nest built by a cowbird. Not a single one is hatched by a cowbird. And not a single cowbird baby is fed and raised by a cowbird. Female cowbirds do hang around the nest sites and lead their young away once the energy-intensive work of rearing them to flying size has been accomplished.
At “The Birds of America Online” (a site sponsored by the Cornell Lab of Ornithology and the American Ornithologist’s Union) it is noted that: “Impact on host species depends on how distribution and abundance patterns of host and cowbird match. The red-winged blackbird, likely North America’s most common species, is an important cowbird host though sheer numbers, even though the percentage of nests parasitized is low. At the other extreme, Kirtland’s warblers produce few cowbirds, although its own existence is actually threatened by brood parasitism because such a high percentage of its nests are parasitized.”
In our region wood thrush, yellow warbler, red-eyed and yellow-throated vireo, ovenbird, American redstart, phoebe, and indigo bunting nest sites are favorite targets of female cowbirds.
How did cowbird brood parasitism evolve? Some ornithologists conjecture that the bird once followed roving bands of bison to feed (then being known as “buffalo birds”) so that they had little or no time to nest in one spot. It therefore became expedient to simply lay their eggs along the way in the nests of other birds. With the demise of the bison herds, the cowbird shifted its attention to cows, thereby spreading east from the great prairies into farming areas. If you visit a dairy operation or other place where there are cows, you’ll find cowbirds. But they’re not all that particular these days — you’ll also find them in low-elevation towns and open woodlands throughout the Smokies region.