Expert consulted: 
With spring well past sprung and summer almost here, baby critters of all types, from deer and rabbits to fledgling songbirds, may be visiting your yard. Maybe you’ve even spotted a nest or two. And tucked among the others, you may have noticed one odd-looking egg that doesn’t quite belong: a cowbird egg.
Brown-headed cowbirds, in case you’re unaware, are one of the continent’s only “brood parasites.” According to Holly Grant, a project assistant at the Cornell Lab of Ornithology, the slightly creepy-sounding term refers to birds who lay eggs in other birds’ nests and do not build their own. “Host species will raise the egg/young as their own, while the cowbird goes on to parasitize other nests,” she explains.
The problem (at least as far as other species are concerned) is that cowbird eggs require a shorter incubation period than most other songbirds and so usually hatch first. The nestlings also tend to grow more quickly than other baby birds.
“These advantages allow them to command the most food from their foster parents, usually resulting in reduced nesting success of the host species,” Holly says.
No one wants to see the nests of favorite winged wonders like bluebirds and orioles fail, which means you might be tempted to interfere when you find a cowbird egg in another species’ nest. But you shouldn’t—for loads of reasons, including that it is flat-out illegal to remove a cowbird egg. Cowbirds are native to North America, Holly explains, and their eggs are protected by the Migratory Bird Treaty Act.
“In very rare cases, such as for black-capped vireos and Kirtland’s warblers, specially permitted individuals may be able to remove the eggs from those two species’ nests because they are ‘near threatened,’” she says. “Otherwise, it’s best to let the cowbird eggs lie.”
Now that you know to leave a cowbird egg alone, here’s what else to know about these birds and their curious nesting behavior.
Some birds reject cowbird eggs—but there can be consequences
There are plenty of species that don’t need any help identifying (and rejecting) cowbird eggs. According to the Georgia Department of Natural Resources, birds like gray catbirds, blue jays, brown thrashers, and American robins will remove their rival’s egg or abandon their nests altogether when one appears. Other birds, such as Eastern phoebes and yellow warblers, will build new nests on top of old nests containing cowbird eggs. However, these acts can result in some nasty payback, notes a report cited by the National Audubon Society.
The 2007 study documented what researchers termed “mafia behavior” by cowbirds (yes, really). In more than half the cases when researchers removed cowbird eggs from warbler nests, mother cowbirds returned to wreak havoc, demolishing the nests and destroying most, if not all, of the warbler eggs. In comparison, only around 6 percent of parasitized nests were ruined when the cowbird eggs were allowed to remain. Tony Soprano would be proud.
How cowbirds lay eggs in others’ nests—and why
The female cowbird’s quest to find the perfect nest for one or two (or occasionally more) of her eggs begins with her searching for other birds who are actively laying eggs, according to Holly. She notes that host species generally range from small gnatcatchers and kinglets to larger meadowlarks, with the cowbird often choosing yellow warbler, song sparrow, and red-eyed vireo nests to parasitize. The cowbird has been found to make use of the nests of more than 220 species, though only 144 have actually been known to rear cowbird young.
“Once she has found a suitable host,” Holly says, “the cowbird will sneak onto the resident bird’s nest when it is away, usually damage or remove one (or more) egg, and replace that egg with one (or more) of her own.” Cowbirds never make their own nest or feed their own young, although Holly adds that females can lay 30 to 40 eggs on average in a summer, and as many as 70 in that same period when food is exceptionally plentiful.
It’s believed that this behavior may have developed as cowbirds followed bison herds across the ancient plains, dining on insects the herds stirred up. “They couldn’t afford to spend a month rearing young when the herd would move on well before they were done,” Holly says. “But, it’s unknown if the nesting behavior developed because of this feeding behavior, or if the birds were able to feed this way because of this nesting behavior.”
Signs you’ve spotted a cowbird egg
It may be possible to spot brown-headed cowbird eggs in a nest since they’re sometimes larger than other species’ eggs and may look noticeably different. (Cowbird eggs are whitish and speckled or streaked with brown or gray.) You might also spot broken eggs in a parasitized nest, or intact eggs below it that the female cowbird has pushed out to make room for her own.
But that doesn’t mean you should intervene in Mother Nature at work. As Holly says, “Many folks are protective of the birds that nest on their property and don’t like to see when they fail. Those are totally fair and valid feelings, but this is not a conscious choice by the cowbirds—it’s the only way they know how to breed.”
Of course, you can always try to discourage cowbirds from visiting your yard by using feeders made for smaller birds. Holly recommends tube feeders that have short perches, smaller ports, and no catch basin on the bottom. You should avoid using platform trays, spreading food on the ground, and dispensing favorite cowbird noshies like sunflower seeds, cracked corn, and millet. “Offer nyjer seeds, suet, nectar, whole peanuts, or safflower seeds instead,” she says.
Or, even better, learn to accept and live with this fascinating species.
“Brown-headed cowbirds are just existing the only way they know how,” Holly says. “Thankfully, because they are native, their host species have evolved along with them over millennia and have had ample time to develop their own adaptations and behaviors to help combat the occasional competition introduced to their nest. Brood parasitism is a natural part of the ecosystem … and cowbirds haven’t been known to be the cause of any major declines in North American songbird species.”
Holly Grant is a Project Assistant at the Cornell Bird Lab on the NestWatch project, and has expertise in nesting birds, feeder birds, and bird feeding strategies among other things. She holds a B.S. in Conservation Biology. You can find out more about Holly on her staff bio page.
