The Brown-headed Cowbird ranks among North America’s most notorious birds; to some ornithologists and conservationists, its name is basically a dirty word. Given its reputation as a brood parasite that in some cases may threaten endangered songbirds, a layperson might assume this member of the blackbird clan is an exotic invader such as the European starling or house sparrow. Actually, the Brown-headed Cowbird is a native of the continent, albeit one that’s dramatically expanded its range in the wake of Euro-American settlement.
Some may take offence at its lifestyle, but we can also appreciate the Brown-headed Cowbird for the unique and surprisingly complex manner with which it perpetuates its kind – and for other intrinsic qualities, including its apparent savvy and the ancient North American ecological system it represents. Read on to find out what we’re referring to on all counts!
Appearance and identification
The Brown-headed Cowbird can be distinguished from other North American blackbirds – with which it often consorts in mixed flocks – primarily by its bill and the head coloration of the adult male.
Compared with other blackbirds, cowbirds have short and stout beaks. Their tails are also comparatively shorter than most of their relatives.
Male Brown-headed Cowbirds are fine-looking birds with black, green-glossed bodies and a rich brown head (surprise, surprise). The female is, as usual, drabber, with a light brownish or grayish body and a streaked belly; its most distinguishing characteristics besides its overall light, earth-toned plumage are the sharp, stubby beak and a black eye. Juvenile cowbirds are more heavily streaked and have a “scaly”-looking back courtesy of white-rimmed feathers.
The Brown-headed Cowbird is a bit smaller than other blackbirds: about 7.5 to 8.5 inches long with a wingspan of about a foot or a bit more. They generally weigh on the order of 1.3 to 1.5 ounces.
In the U.S., the Brown-headed Cowbird overlaps with two more tropical cousins. In South Florida, you may have to distinguish between wintering Brown-headed Cowbirds and the similarly sized Shiny Cowbird, but that’s pretty straightforward: The Shiny has a thinner bill, females are less streaked and somewhat darker than their Brown-headed counterparts, and the striking males are a uniformly glossy, indigo- or blueish-black.
In the Southwestern borderlands, meanwhile, the Brown-headed Cowbird crosses paths with another cousin, the Bronzed Cowbird, which is quite different-looking: It’s a larger, stockier bird with a heavy head, a thick, slightly curved bill, and darker plumage, but the unmistakable feature is the brilliant red iris.
Range and habitat
Originally the Brown-headed Cowbird was primarily an inhabitant of the shortgrass prairies of the North American Great Plains, where it trailed American bison to snatch up insects flushed by the great herds. Major changes wrought by Euro-American occupation of the continent allowed the cowbird to hugely expand its domain: Conversion of eastern forests to agricultural and residential land gave this open-country bird its preferred habitat, and the proliferation of cattle and other livestock gave it new “herds” to follow.
According to Catherine P. Ortega’s Cowbirds and Other Brood Parasites (1998), Brown-headed Cowbirds had colonized parts of the Northeast as early as the late 1700s; they were slower to spread into the Southeast and the Far West. Today, Brown-headed Cowbirds inhabit all of the Lower 48 states either as year-round resident (most of the East, South, and West Coast), a breeding visitor (the Upper Midwest and most of the West as well as parts of southern Canada), or a wintering bird (central and southern Florida).
As we’ve already suggested, you’re unlikely to find a Brown-headed Cowbird in dense woods – or, at least, unlikely to linger in dense woods, for evidence suggests cowbirds will enter heavy forest in search of host nests. Generally speaking, the species prefers open grassland, fields, pastures, shrublands, marshes, and suburban and otherwise developed landscapes.
Brown-headed Cowbird diet
Brown-headed Cowbirds primarily eat seeds – which dominate the winter diet – and insects, which may be roughly half the bird’s fare in warmer months. Behind a bison, cow, sheep, or horse, a cowbird snatches grasshoppers, leafhoppers, beetles, and other insects stirred up by the big grazer’s passage. (Have you figured out how the cowbird earned its name? Although throughout most of its evolutionary existence, it would have better been called a “Bison-bird.”)
During the nesting season, female Brown-headed Cowbirds will supplement their menu with snail shells and other birds’ eggs to provide calcium for their prodigious egg-laying.
Communication and call song
The male Brown-headed Cowbird sings in a liquid warble of gurgles and whistles. Both males and females call with liquid, ratchety chatter, and in flight the birds make a seet whistle.
You can listen to the Brown-headed Cowbird call sound here.
Nesting and mating
The most famous – or shall we say infamous – characteristic of the Brown-headed Cowbird is its nesting habits, or rather lack thereof. The species is a brood parasite: a bird that lays its eggs in another’s nest and relies on this “host” to raise its young.
Before we get to that fascinating aspect of behavior, though, we need to cover the whole mating deal. Males display for females by singing their fluid songs while puffing up their body feathers, fanning out wings and tails, and bowing; multiple males may perform this ritual in close quarters in an attempt to win the attention of lady cowbirds. How monogamous or not Brown-headed Cowbirds are isn’t very clear.
Eggs and young
A female Brown-headed Cowbird may produce dozens of eggs in one breeding season – more than 70, in fact, have been recorded. Her strategy is to identify “hosts” by keying into the nest-building or nest-maintenance activities of other songbirds, or by actively flying around trying to flush them from their broods. Brown-headed Cowbirds parasitize a whopping number of host birds: Their eggs have been found in the nests of more than 200 other bird species, though the number of regular hosts that actually raise cowbird young has been estimated at closer to 130. Frequent hosts range in size from tiny kinglets to cardinals and meadowlarks. Some studies have suggested individual cowbirds may tend to specialize in a particular species of host.
Female cowbirds lay one or two eggs in a host nest around dawn, often when the host bird is absent; commonly, however, the cowbird flushes the host from the nest to lay, and may endure attacks from it in the process. The cowbird frequently removes one of the host eggs from the nest, but this is often done on a visit prior to actually laying its own; it may or may not consume this pillaged egg. Some studies suggest cowbirds are more likely to remove an egg from the nest of a larger host, such as a blackbird or cardinal – perhaps to reduce competition for their young.
Nestling cowbirds in general have a leg up on their nest-mates given their larger size; this allows them to outcompete the host’s young for food, and sometimes fling them from the nest or smother them. (It’s a cold, hard world out there, huh?) Scientists aren’t quite certain how young cowbirds learn to be cowbirds, given their foster environment, but a 2015 study that outfitted adult cowbirds and their offspring with radio transmitters showed juvenile cowbirds leaving their nest at night to roost in fields – aka typical cowbird habitat – and returning the next day. It’s possible these young cowbirds are interacting with and imprinting on their own kind, including adult cowbirds, at these night roosts.
The same research showed mother cowbirds don’t entirely abandon their young once they’ve laid eggs in a host next, but rather periodically check in – perhaps assessing the relative success or failure of the parasitism to decide where to plant eggs in the future.
Interestingly, research has also suggested that female cowbirds may ravage host nests from which their eggs have been removed (an anti-cowbird strategy some parasitized birds pursue) in what’s been termed retaliatory “mafia” behavior. Some evidence suggests cowbirds may also destroy nests they haven’t laid eggs in as an attempt to force the hosts to build new ones for the cowbird to parasitize.
The brood-parasite strategy the Brown-headed Cowbird pursues is completely natural (and one shared by a variety of other birds, including many cuckoos and honeyguides). When the cowbirds parasitize endangered or threatened songbirds, though, the behavior may be a conservation risk. In some cases, certain agencies or organizations such as the U.S. Fish & Wildlife Service or the Audubon Society may receive permission to control cowbird populations in order to support avian species of concern, such as the Kirtland’s Warbler and Bell’s Vireo.
Sources
- Cornell Lab of Ornithology: All About Birds: Brown-headed Cowbird.
- Davies, N.B. 2000. Cuckoos, Cowbirds and Other Cheats. A&C Black Publishers Ltd.
- Lewis, Amy. 6 June 2018. “Is It Okay to Remove Cowbird Eggs From Host Nests?”Audubon.
- Ortega, Catherine P. 1998. Cowbirds and Other Brood Parasites. The University of Arizona Press.
- Sibley, David Allen. 2000. The Sibley Guide to Birds. Alfred A. Knopf.
- Soniak, Matt. 25 February 2016. “How Does a Cowbird Learn to be a Cowbird?”Audubon News.
