One sunny spring morning, a small flock of Brown-headed Cowbirds, accompanied by several Common Grackles, dropped into my back yard. I felt amazed because I'd never seen them in my urban garden space during many years of feeding backyard birds. I felt dismayed because Brown-headed Cowbirds have an unsavory reputation for committing reproductive fraud and deceit.
With stealth and cunning, female cowbirds steal into other species' nests when the owners are away and deposit eggs. The parents, unaware of the trickery, might raise the cowbird chicks at the expense of their own, a feat that happens across the North American landscape. I thought that our backyard birds could be at risk but didn't see the cowbirds again and thought no more about it during the breeding season.
Dropping eggs into the nest of another species is not uncommon in the avian world. This behavior, brood parasitism, occurs in two, very different ways. Facultative parasitism often occurs among ducks, geese and colonial nesting birds when females deposit eggs in neighboring nests and the unsuspecting parents raise the hatchlings. With this type of parasitism, female depositors build nests and raise broods but occasionally donate eggs to neighbors in dense breeding colonies on island cliffs, ocean dunes or wetlands.
Offspring of facultative females usually are assimilated into the broods of unsuspecting parents and raised with little disruption or negative consequences to the adoptive family. Although most facultative species are wetland or colonial birds, the range of species includes Black- and Yellow-billed Cuckoos and gamebirds such as Ring-necked Pheasants and Northern Bobwhite.
Brown-headed Cowbird, an obligate brood parasite, is a completely different story and far less benign to its breeding neighbors. Female Brown-headed Cowbirds do not build nests, and after covertly dropping eggs into a nest, they move on without further regard for the offspring.
Only about 1 percent of all avian species worldwide are obligate brood parasites, and Brown-headed Cowbird is the only obligate common to North America. Its parasitic breeding behavior probably results from early migratory foraging behavior throughout the grasslands of North America.
For eons, Brown-headed Cowbird flocks followed vast herds of buffalo, dining on the grasshoppers and other insects kicked up by the buffalo. Because the cowbirds didn't stay in one location long enough to build nests and raise the offspring, they began depositing eggs in the nests of "sedentary" species that nest at the forest edges as well as grasslands.
Before the invasion of European pioneers, the species' range included the prairies between the eastern hardwood and the southern pine forests as well as the western mountain ranges. Explorers who found the cowbirds accompanying the migrating buffalo herds called them buffalo birds.
Early explorers found the Cumberland Gap in the 1750s, a passageway through the Appalachian Mountains that provided a gateway to the Midwest prairies. Soon, pioneers spilled onto the plains, and as settlement pushed westward, adventurers decimated the buffalo herds. Herds of cattle and agricultural crops replaced the buffalo, and Brown-headed Cowbirds, sometimes called cowpen birds or cow buntings, adapted to the changing landscape.
As prairies turned into croplands during the early 1800s, eastern forests shrunk to make room for towns amid mushrooming farmland, and cowbirds greatly expanded their range. By the late 1800s, the species appeared across North America, later reaching Southern California around 1900, Nova Scotia in the 1930s, and Georgia and Florida in the late 1950s.
Whether at the forest edge, on a grassland fencepost or watching over a suburban backyard, Brown-headed Cowbird females usually strike just before dawn. Most often, the cowbird pair stakes out the locale to identify one or more likely host targets.
Brown-headed Cowbirds parasitize roughly 200 species throughout North America, and 144 species have raised cowbird offspring. About 50 species are known as reoccurring hosts. Female cowbirds can distribute up to 50 eggs among various species in their territory.
The cowbird couple might use several stealth techniques. If the nest-building or incubating parents are foraging, the female might dart into the unattended nest and deposit one or two eggs. If there is sufficient time, she may crush and jettison host eggs that would compete with hers.
When a nesting parent appears, the male cowbird harasses it until the parent rises to the challenge and chases the male cowbird from the nest. The female secretly waits for the chase to begin and then swoops in to deposit her eggs.
Many species accept the foreign eggs in their nests and incubate them. Many grassland, woodland and wetland species readily accept cowbird eggs, including Red-eyed Vireos, Yellow Warblers, Eastern Phoebes, Song Sparrows and Orchard Orioles. Cowbird eggs have an incubation period of 11 to 12 days, one or two days earlier than most host species. Cowbird hatchlings then begin their sojourn with a head start over host chicks.
Cowbirds' Toll In The Nest
Raising cowbird hatchlings among a brood places the host species' chicks at risk. The interloping chicks generally are larger and much more demanding than host hatchlings, and their mouths are larger targets for food brought to the nest. Unable to compete, host nestlings often perish. This is especially true for smaller host chicks such as Blue-gray Gnatcatchers, Indigo Buntings, Lark Sparrows and Yellow Warblers.
The toll that cowbird chicks take on resident nestlings is immense for some species. Field studies have shown that only 28 percent of Chipping Sparrow chicks survive in a parasitized nest, and the figures are even bleaker for Eastern Phoebes, Kirtland's Warblers and Common Yellowthroats, all at or below a 10 percent survival rate. In general, smaller species suffer greater chick mortality or lose an entire clutch with a cowbird hatchling in the nest.
The chicks of larger species, such as meadowlarks and Red-winged Blackbirds, usually fare much better because they can compete for their share of food, even with a more persistent and noisy nest mate. Overall, survival rates are much higher for larger species than smaller ones.
Accept Or Reject?
A number of species seem to recognize a foreign object placed in their nests and typically reject Brown-headed Cowbird eggs soon after discovery. Many species reject foreign eggs some of the time, and several species consistently reject the eggs, such as American Robins, Gray Catbirds, Eastern Kingbirds, Blue Jays, Northern Orioles, Cedar Waxwings, Brown Thrashers, Crissal Thrashers and Sage Thrashers.
Species that reject cowbird eggs recognize the eggs' different color or size and throw them out of the nest. Some species, like Bullock's Orioles and American Robins, pierce or crush the eggs before jettisoning the remnants. Others--such as meadowlarks, Dickcissels, Cedar Waxwings and Indigo Buntings--abandon the nest and eggs and build another one.
Although Yellow Warblers often accept a cowbird egg, they have an unusual, but not always successful, defense against parasitism. Upon detection of a foreign egg, parents place a layer of nesting material over all the eggs before starting a new clutch. When threatened with successive cowbird deposits, Yellow Warblers might install several layers or desert the nest. A few years ago, a nest discovered in Ontario, Canada, contained six layers built over eggs laid during various cowbird visits.
Why do so many more species accept cowbird eggs than eject or crush the eggs or flee to another nest site? Perhaps smaller species lack the capability to roll foreign eggs out of their nests or to destroy the intruder's eggs with their small bills. Such attempts by the smaller species might result in harm to their own eggs, so it is safer to accept the interloper than to jeopardize their clutch.
Perhaps many species have been exposed to cowbird parasitism only over the past 200 years--an insufficient time to develop effective defensive strategies. When this "evolutionary lag" catches up, more acceptors likely will become rejectors.
Many people think that the parasitism by Brown-headed Cowbirds has contributed greatly to plummeting numbers of prairie and forest species over the past several decades. How pivotal has this species been in the diminishing numbers of other species? Kirtland's Warbler gives us a glimpse of how nature and human intervention unwittingly conspire against certain species.
Kirtland's Warbler, an endangered species, nests on 30,000 acres in Michigan and winters in the Caribbean Islands. Its numbers have declined steadily, and Brown-headed Cowbirds were thought to be primary culprits.
In the early 1970s, alarm among scientists stimulated a vigorous effort to control cowbird numbers. Since 1972, more than 120,000 cowbirds had been trapped and killed. During the first decade of cowbird removal, parasitism dropped to just 3.4 percent of Kirtland's Warbler nests, and three times as many warbler hatchlings survived as before the removal of the Brown-headed Cowbirds. The number of breeding pairs, however, stayed about the same--roughly 200 pairs--despite the use of aggressive anti-cowbird measures.
In 1981, a prescribed burn near the Kirtland's Warbler breeding grounds got out of control, and the wildfire scarred 24,000 acres of pine forest. The fire created more suitable habitat, and the number of breeding warbler pairs began to increase. The number of singing males increased because of additional habitat.
Although Brown-headed Cowbirds place reproductive pressure on many species, ultimately the primary destructive force behind plummeting avian populations is the continuing loss of habitats to development and urban sprawl across North America. Sprawl leads to loss of habitat and forest fragmentation--and an inevitable increase in familiar pest birds, such as House Sparrows, European Starlings and Rock Pigeons.
No wonder we so often find Brown-headed Cowbirds throughout the countryside and in our own backyards. Despite many years of feeding backyard birds, I had avoided cowbirds in my backyard habitat. Their brief visit that memorable spring morning, however, later produced an unwelcome surprise.
Early one mid-May morn, a Brown-headed Cowbird chick appeared beneath my sunflower feeder--plump and mouth agape. At its side, a Song Sparrow dutifully attended the new arrival. This "Mutt and Jeff" pair foraged around the feeder for several mornings, as the sparrow furiously plucked spilled seeds from the ground and stuffed them into the cowbird's cavernous gullet. Then, quite suddenly, the juvenile cowbird vanished.
The neighborhood gave a collective sigh of relief, aware that our first cowbird visit might not be the last. In the future, our backyard birds might face a threat that lurks just out of sight, waiting for a chance to complicate a host's life and simplify its own.
Excerpt from WildBird March/April 2005 issue, with permission from its publisher, I-5 Publishing LLC.