Brown-headed Cowbirds

Brown-headed Cowbirds have a bad reputation. A lot of birders don’t like them because they seem to be shirking their parental duties. Because they are obligate brood parasites—they don’t build their own nests, and only lay their eggs in the nests of other species—we accuse them of taking advantage of other species by forcing them into doing all the work of feeding a hungry nestling. It’s unfair. We’re indignant.

Some birders even go so far as removing cowbird eggs from any parasitized nests they come across. This is a bad idea for many reasons. For one, it’s illegal, as cowbirds are protected by the Migratory Bird Treaty Act. Then, while many host parents don’t realize that there’s a different egg in their nests, they do know how many eggs should be there. If one is missing, they may abandon the entire nest, or even their territory! Worse, if the cowbird eggs is removed or destroyed, a 2007 study discovered that the parent may come back and totally destroy the host nest. It seems like retribution, but from a natural selection viewpoint, it makes sense; allowing the brood to survive could pass on the ability to recognize a cowbird egg, so those genes must be eliminated.

Besides, cowbirds may not be as evil as we think. Research has exposed some surprising facts that shed new light on the species.

Brown-headed Cowbirds, as their name implies, tend to follow large grazing mammals, feeding on the insects on their hide or found in their droppings. They  likely evolved this behavior as they followed the roaming herds of bison. Because of their nomadic lifestyle, they couldn’t hang around to ensure the survival of their young.

However, it turns out that when possible, the mother cowbird doesn’t abandon her young. Instead, she hangs around, making sure that junior is thriving. The above example of nest destruction is only possible if the mother knows her egg is missing.

How can a Brown-headed Cowbird chick, growing up in a foster family, learn how to be a Brown-headed Cowbird? At first, scientists assumed that mom was involved here too, but a 2015 study revealed otherwise. Instead, the young fledge early and roost away from the nest at night. This could help them avoid imprinting on the songs and behaviors of their host family[i], as well as offer an opportunity for them to meet other cowbirds.

Brown-headed Cowbird_YellowstoneNP-WY_LAH_2636

Finally, unlike European cuckoos, Brown-headed Cowbirds don’t kill their foster siblings. Instead, they join forces, clamoring for food, and driving the harried parents to greater efforts to keep them all fed. Because the cowbird chick hatches sooner and grows faster than its nestmates, it needs that additional food. According to one study, “If you look at the growth of the cowbird chick, it does best in hosts who have … approximately two nestmates growing up together with the cowbird chick.”[ii]

If we vilify cowbirds, we should also blame Eastern and Western Bluebirds, Purple Martins, and Tree Swallows. While not obligate parasites, they have also been known to lay eggs in nests not their own. So do some species of ducks—and even turkeys have been observed dropping eggs in handy chicken nests. Apparently, foster parenting is a successful strategy for passing on ones genes for many species.


[i] https://www.sciencedirect.com/science/article/pii/S0003347215003401
[ii] https://www.berkeley.edu/news/media/releases/2004/08/05_cowbird.shtml

The answer to last week’s quiz is White Ibis.

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