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Brown-headed Cowbird

A bird that never nests has a huge impact on nesting birds


Harold Stiver, http://ontfin.com

How could a bird incapable of nesting, incubating, or raising babies reproduce? The Brown-headed Cowbird found one solution—it lays its eggs in the nests of other birds. More than 220 host species have been parasitized by cowbirds.

Some nests are clearly inappropriate. In the only known case of a cowbird parasitizing a Ruby-throated Hummingbird nest, the egg entirely filled the nest and never hatched. But 144 species have been documented rearing cowbird young.

Female cowbirds search for nests by quietly observing other birds, and also by making hard landings on leafy branches while flapping their wings as if intentionally trying to flush birds off their nests.

When a cowbird lays an egg, it usually tosses a host egg out of the nest. Many hosts don’t seem to notice the replacement, and when they do, if they try to toss out the cowbird’s huge egg with their relatively tiny bill, they sometimes scratch or pierce their own eggs. Also, as Irby Lovette notes in “Extortion Rackets and Egg-Farming by Cowbirds” (Spring 2008 BirdScope Online), when a host bird does remove a cowbird egg, the cowbird may return to destroy the remaining eggs.

Each time a songbird returns to its nest, it feeds the baby with the widest gape, which normally is the hungriest baby. If the cowbird is larger than the other nestlings, it gets the lion’s share. Some host birds may successfully raise both a cowbird and one or two of their own, but in some species, the cowbird is almost always the only survivor.

Neotropical migrants with limited ranges, such as Kirtland’s Warbler, are exceptionally vulnerable to cowbird parasitism. The worst case scenario may be experienced by the Black-capped Vireo, which breeds in scrubland in a very restricted and shrinking range in Oklahoma and Texas. In the 1980s at one Oklahoma site untrapped for cowbirds, 14 of 15 observed nests were parasitized. That entire population is now believed extirpated. But in areas where cowbird parasitism levels are reduced to 3 percent of observed nests, many female vireos successfully fledge two vireo broods in a season.

The Black-capped Vireo has an unusually long incubation period of 14-17 days. Cowbirds hatch in 10-12 days, so one may be 4 or 5 days old by the time the first vireo hatches. In The Birds of North America Online (www.bna.birds.cornell.edu), ornithologist Joseph Grzybowski notes that hatchling Black-capped Vireos have died under the weight of a cowbird nestling. He also notes the case of an adult cowbird killing a seven-day-old vireo and laying her egg atop the carcass. By the time a cowbird fledgling flies off to associate with other cowbirds, the host parents have wasted an entire nesting cycle.

Many ornithologists believe that cowbird numbers were once limited by high winter mortality and scattered bison herds. But when people decimated bison, they introduced cattle and started growing crops such as winter wheat, vastly increasing cowbird winter survival and giving their burgeoning numbers an ever-expanding range.

Today, agriculture, development, and forest fragmentation maintain cowbird numbers at high levels. The most effective way to compensate is to manage cowbirds. In the Wichita Mountains in Oklahoma, wildlife managers have been trapping more than 90 percent of local cowbirds every spring and summer since the 1980s. Here, the Black-capped Vireo population, though precariously localized, now thrives, along with Painted Buntings, Summer Tanagers, Blue-gray Gnatcatchers, and other songbirds that produce many more young every year without nest parasitism.

Laura Erickson

 

For permission to reprint all or part of this article, please contact Laura Erickson, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-1114. email: lle24@cornell.edu

 
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