Say the name Brown-headed Cowbird (Molothrus ater) to a birder and you’re likely to get a scowl or look of disgust. The Brown-headed Cowbird is a species that occurs commonly in the garden during the summer season but is often overlooked.

Members of the blackbird family, males are aptly named, with a chocolate brown head contrasting with an overall black body. Female Brown-headed Cowbirds are often a source of confusion for newer birders, as they are a quite nondescript gray-brown overall. A helpful key to identifying them is their conical, straight-edge bill and just their overall uniform coloration. Both male and female cowbirds are often seen perched high on bare branches, not unlike a typical flycatcher. However, unlike flycatchers, they are not on the lookout for insect prey to grab. Cowbirds are primarily ground-feeding seed and arthropod eaters. So why do they spend time perched in the open, and why do they have such a bad reputation? 

Brown-headed Cowbirds are among a group of species known as brood parasites. Rather than building their own nests and raising their own young, brood parasites lay their eggs in the nests of other birds and allow these foster parents to raise their young. If you’ve ever observed a small bird, like a Chipping Sparrow, busily feeding insects to a much larger nondescript brownish begging bird, you’ve observed this phenomenon. Brown-headed Cowbirds have evolved several strategies that make them particularly effective at this behavior.

Below: A chipping sparrow feeding a cowbird chick. Photo - @Christophe Buidin.

Cowbirds are often seen perched on a bare branch keeping an eye out for nesting activity of a suitable host. The female cowbirds are quite adept at waiting until the parent host has left their nest unguarded and then quickly adding an egg to the clutch. One female cowbird can lay as many as forty eggs in a season. Their eggs have a relatively short incubation period and consequently often hatch before the host’s young. These newborn cowbird chicks will then destroy the host’s eggs by pecking holes in them or even pushing them out of the nest. If the host’s eggs have hatched, the larger cowbird can bully its way to grabbing the majority of the food brought by the parent. While such behavior may seem repulsive, we have to remember that these traits have been selected to make the Brown-headed Cowbird very successful at what they do. 

Several very interesting questions arise from the lifestyle of the cowbird and its host. Among the most interesting is how this behavior evolved in the first place. One of the leading theories is that Brown-headed Cowbirds evolved to follow bison herds as they roamed the prairies, taking advantage of the insects that were attracted to or stirred up as the bison grazed. Because bison herds constantly moved, the cowbirds could not simultaneously remain in one location to nest and stay with the bison herd. So they adapted by allowing other birds to take over parental responsibilities. Another intriguing question is why the host birds do not recognize the foreign egg or hatchling as an intruder. In fact, some species of birds do detect the alien in their midst. For example, some birds have been known to build a new nest on top of an existing one with a cowbird egg, even if it means abandoning their own eggs. They then proceed to start over with a new clutch of eggs. The record for this behavior was set by a particularly persistent Yellow Warbler that successively built six new nests on top of the original one. Hopefully, they were successful in the seventh try. 

On the other hand, many birds do not seem to recognize that they are incubating or feeding an intruder. This can have an impact on the breeding success of some species that are of concern. European colonization of North America has had an unfortunate consequence, as bison herds were decimated and forests were cleared, creating more edge habitat preferred by the cowbird. As a consequence, the balance between host and brood parasite has been altered in a way that favors the latter. In some cases, biologists have intervened by establishing active Brown-headed Cowbird trapping programs to try to protect especially vulnerable species, such as the endangered Kirtland Warbler that breeds in Michigan.

So what are we to think of the Brown-headed Cowbird? Is it the villain that it is so often portrayed as or just a part of nature that we should observe and appreciate? Will you give a sneer the next time you see a cowbird in the garden or marvel at its unique evolutionary strategy to survive? Last summer I found a large speckled egg in the nest of a House Finch that had taken residence in a hanging pot on my porch. Should I have left the egg or intervened? I’m not really sure myself!❖ 

Howard Towle has actively birded at Wirth Park for over 30 years, having seen more than 160 species in the park. He is a Garden volunteer and Friends supporter. This article was originally published in The Fringed Gentian™ in Fall 2024. Photos with a “CC” credit are used for educational purposes under Creative Commons license. Learn about this at https://creativecommons.org/share-your-work/cclicenses/