July/August Bird of the Month: Brown-headed Cowbird

by Elise Brosnan

Brown-headed Cowbirds love cows. True to their name, they can often be found alongside herds of cattle or horses, eating insects that much larger animals flush from the grass. Historically, Brown-headed Cowbirds followed bison herds across the Great Plains, but the spread of livestock farming has expanded their range across North America. 

Brown-headed Cowbird, Allison Farrand

Brown-headed Cowbird, Allison Farrand

Their arrival has been unfortunate news for many species of birds. Brown-headed Cowbirds reproduce by being brood parasites; instead of nesting and raising their own young, they lay their eggs in the nests of other bird species, which the hosts then raise until adulthood. 

Most brood parasites are specialists, meaning they lay their eggs in the nest of only one other bird species. These eggs closely mimic the hosts’ eggs, and their hatchlings closely mimic the hosts’ hatchlings. Because their young make for convincing changelings, the hosts are tricked into caring for them.

However, Brown-headed Cowbirds are generalists and are known to have parasitized over 200 species of birds. Most of these birds have young that look nothing like Brown-headed Cowbirds, so why do they choose to raise offspring that are clearly not their own? It turns out that Brown-headed Cowbirds are even bigger bullies than previously thought.

In what researchers call “mafia behavior,” birds that eject Brown-headed Cowbird eggs may be subject to retaliation. In one study, researchers identified several hundred Prothonotary Warbler nests, some of which had been parasitized by Brown-headed Cowbirds. They removed the cowbird egg from some of these parasitized nests, and then protected some of the nests from harassment by cowbirds.

When a cowbird egg was removed from an unprotected nest, the nest had a 56 percent chance of being destroyed, including any eggs or nesting chicks. When the cowbird egg stayed in place, an unprotected nest had only a 6 percent chance of being destroyed. All of the protected nests were undisturbed. Because of this behavior, Prothonotary Warblers that raised a Brown-headed Cowbird had 60 percent more reproductive success than Prothonotary Warblers that had a cowbird egg removed from their nests. Brown-headed Cowbirds are unwelcome guests, but kicking them out may make them even worse.