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 cousin the shiny cowbird, a South American species that appears in the Deep South, and the western bronzed cowbird, 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 its 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.
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.