Female Brown-headed Cowbird. Calumet Park, October 20, 2009. Photo by Walter Marcisz.

words by Lauren Kostas

The first Friday in September is Lazy Mom’s Day, a holiday that invites harried moms to enjoy some well-earned “me time.” Moms are encouraged to take a break from busy schedules and myriad demands to stroll a neighborhood park, enjoy a relaxing bubble bath, order take-out dinner for the family, or curl up with a good book like Amy Tan’s Backyard Bird Chronicles.

One Chicago summer resident has elevated Lazy Mom’s Day to a complete lifestyle, permanently offloading all mothering responsibilities to others. The female Brown-headed Cowbird (Molothrus ater) eschews the burden of nest building and chick rearing by depositing her eggs into the nests of other birds. The unsuspecting host parents incubate the eggs and raise the cowbird nestlings as their own. This rare behavior (only 1 percent of all bird species) has earned the cowbird a name worthy of a Korean horror film—brood parasite.

The female Brown-headed Cowbird is brownish gray, with a short tail, dark eyes, and a sturdy conical bill. Cowbirds arrive in Chicagoland in February, favoring open areas such as fields and lawns. Egg laying begins in April, when the females shop the forest edges for available nests into which they deposit their white eggs with brown markings. Cowbirds are open to all styles of nests—from dome-shaped nests on the forest floor to cup nests in shrubs and trees. Unburdened by raising young, female cowbirds are prolific egg producers. Individuals generally lay an egg a day until a clutch has been produced and placed in as many different nests as there are eggs. These so-called songbird chickens can lay as many as three dozen eggs each summer. At least 61 species of Illinois-nesting birds have been parasitized by the cowbird, including the Yellow Warbler, Song and Chipping Sparrows, Red-eyed Vireo, and Red-winged Blackbird.

Juvenile Brown-headed Cowbird. Calumet Park, September 7, 2019. Photo by Walter Marcisz.

Cowbirds have evolved behaviors that set their young up for success without any interference or support from them. Cowbirds appear to choose the nests of smaller bird species. The dropped off egg is generally bigger than those of the host and hatch days faster (11–12 days versus 12–14 days). While cowbird chicks don’t push their “siblings” from the nest like some cuckoo species, they tend to grow faster and out compete their smaller nestmates for “parental” attention, food, and space. You may have seen a diminutive Yellow Warbler or vireo parent struggling to keep up with the appetite of their cowbird nestling.

A few host species do recognize the interlopers and act. They may remove the cowbird egg from the nest, desert their nest, or even build a new nest on top of the one containing the cowbird egg. Researchers from the University of Illinois are studying how American Robins sometimes identify and eject cowbird eggs.

Cowbirds are often unfairly villainized for their reproductive strategy. But their distinctive approach to parenting is a natural process with an important place in a healthy ecosystem. Some scientists argue that studying Cowbirds and brood parasitism can uniquely help us understand avian social behavior, physiology, evolution, and conservation.

Male Brown-headed Cowbird. LaBagh Woods, May 10, 2021. Photo by Nathan Goldberg.

For instance, given the apparent lack of cowbird parent/offspring interaction, one question behavioral scientists are exploring is how young cowbirds know that they are in fact cowbirds. Before their first winter, cowbirds leave their foster species to join other cowbirds. Research is exploring how sight and sounds seem to trigger the juvenile cowbirds to recognize their own species. Nestlings are observed to react to the sounds of their own species and distinguish the chatter of an adult cowbird. As the saying goes, “Birds of a feather flock together,” and so do juvenile cowbirds choose to associate with adults that look like them. Researchers marked the feathers of some young cowbirds and found that these birds chose to associate with adult cowbirds that bore the same artificial markings.

There is a lot to learn from this fascinating, somewhat maligned, native species for which Lazy Mom’s Day is every day of the year.

For more information on the Brown-headed Cowbird, check out the Birds & Bytes episode “The Notorious Cowbird,” presented by researcher Sarah Winnicki.

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