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Bird's-Eye View: Having their young without any responsibility
U.S. Fish and wildlife Service Photo
Brown-Headed Cowbirds take advantage of other birds’ nesting sites.
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Published: November 4, 2008
Updated: 11/03/2008 08:10 pm
The Brown-Headed Cowbird, a common bird throughout the United States, is frowned upon by many birders because it is a brood parasite.
A brood parasite does not build a nest, incubate its eggs or raise its young like other birds. Instead, it finds some unsuspecting bird that is nesting, lays its eggs in that bird's nest and flies off without the slightest notion of ever caring for its own young. No incubating, no feeding, no staying up nights with colicky babies, no college fund to worry about. Nothing.
To make matters worse, the female cowbird, as she is about to lay her eggs in another birds nest, will often remove one of the host bird's eggs, carry it off and sometimes eat it, partly to restore the calcium lost from producing her own eggs.
And it hasn't chosen just one hapless species to serve as a surrogate parent. According to the Cornell Laboratory of Ornithology's "Birds of North America," an electronic field guide, the nests of 220 different bird species have been parasitized by Brown-Headed Cowbirds, and 144 of those species have actually reared cowbirds.
Many of the host species are much smaller than cowbirds. Host birds usually weigh from 8.3 to 18 grams as adults. When the young cowbirds leave the nest, they are nearly the size of their true parents, approaching 44 grams. Thus they are two to five times the size of the birds that raised them. And you think your teenager eats a lot.
Some aren't fooled
Some birds are wise to the habits of cowbirds. They respond to the alien eggs in different ways. Some will simply eject the foreign egg from their nest. Others will build a new floor to the nest over the unwanted cowbird egg. And if the cowbird lays another egg on the new nest floor, the host bird may build yet another floor, covering up the cowbird's second attempt at deception. There are records of birds building third and fourth layers to their nests trying to avoid cowbird nest parasitism.
But many species do not recognize the difference between the cowbird's eggs and their own, even though the eggs may differ in size, color and pattern.
Birders take offense at the cowbird's biology -- not just out of moral indignation -- but because these practices take a toll on the populations of many songbirds, while cowbird populations thrive.
Cowbird eggs tend to hatch a little earlier. This gives the cowbird nestling a head start on its nest mates. Because of this head start and the larger size of the cowbird nestlings, they tend to get the lion's share of food provided by the surrogate parents. Consequently, the other nestlings may be stunted in their growth because of inadequate food. Some of them even starve.
But does this bird really deserve our contempt? Maybe not.
It is thought that the cowbird developed a commensal relationship with the American bison long before cattle were on the landscape. These large herbivores traveled the prairies in vast herds and stirred up huge numbers of insects. Cowbirds became dependant on this food resource and developed brood parasitism as a means of coping with a traveling banquet. The bison were constantly on the move to greener grasses. and the cowbirds needed to follow them to keep up with their food; they couldn't stay put to build nests and raise families. Laying their eggs in the nests of other birds ensured that cowbird populations would endure while the adults could carry on with their pursuit of insects stirred up by the constantly moving bison herds. These behaviors, along with vocalizations and courtship behaviors, are thought to be inherited, as the young birds have no opportunity to learn them from their parents.
Bison are grassland-dependent, therefore cowbirds are mostly grassland species and penetrate only into the edges of forests in search of bird nests. Before the arrival of Europeans, North American forests were vast and relatively undisturbed. Most forest-bird species were not parasitized by cowbirds. But over the last 200 years, we have divided and subdivided forests so much that it's easy for cowbirds to penetrate the resulting patches of woodlands and reach the nests of forest-dwelling birds. There are probably far more cowbirds and far more bird species parasitized by them than there were prior to the 1800s. But we made that possible by modifying the landscape. The cowbirds just took advantage of the opportunities we gave them.
So, whether you object to the habits of the cowbird or admire this unique reproductive strategy, you must admit -- it is a pretty interesting bird.
□ The Audubon Society of Forsyth County will hold a bird walk at 9 a.m. Saturday at the Muddy Creek Greenway. The greenway is in western Winston-Salem, just off Meadowlark Drive between Country Club Road and Robinhood Road.
Meet in the gravel parking lot behind Meadowlark Elementary and Middle schools.
■ Bird's-Eye View is a joint column by Ron Morris and Phil Dickinson. Today's column was written by Morris. Morris retired after 24 years as curator at the N.C. Zoo. He has studied birds around the world and is currently the vice president of the Audubon Society of Forsyth County. Dickinson is a legal writer. He has been an active birder for 15 years, and is a past president of the Audubon Society of Forsyth County and is the chairman of the conservation committee. If you have a birding question or story idea, write to Bird's-Eye View in care of Features, Winston-Salem Journal, P.O. Box 3159, Winston-Salem, N.C. 27101-3159, or send an e-mail to birding@wsjournal.com. Please type "birds" in the subject line.
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