Mysterious Marvels or Villainous Vagabonds?

By Lynette Anderson, Belwin Conservancy Naturalist
Photo by Bear golden retriever from Auburn, N.Y., via Wikimedia Commons.

Brown-headed Cowbirds (Molothrus ater) are a member of the Icteridae family, which includes all species of Blackbirds as well as Orioles, Meadowlarks and Bob-o-links. They are a stocky bird with a stout, short bill and can be found in any open pasture, urban yard or grassy forest edge. They are historically a bird of the open spaces, following the roaming bison herds over the wide prairies.

As traveling vagabonds, nesting and raising a family is out of the question for these birds. They are the only species in North America that parasitizes, or lays its eggs, in other birds’ nests, leaving the young to be raised by the unsuspecting foster parents. This trait might help increase their chances of survival in the wild but it’s earned them a bad reputation with many birders.

Brown-headed Cowbirds eat mainly seeds from grasses with some crop grains. Insects such as grasshoppers and beetles make up about 25 percent of the Cowbird’s diet. Females have a large calcium requirement due to their laying of up to 40 eggs in a nesting season. To satisfy this high need, they eat snail shells and eggs from other birds’ nests they visit.

An Eastern Phoebe (*Sayornis phoebe*) nest with one Brown-headed Cowbird (*Molothrus ater*) egg. Photo by Galawebdesign via Wikimedia Commons.

As breeding season begins, while the males are showing off for dominance and mating rights, the female is quietly observing other, usually smaller birds like Thrushes, Warblers and Vireos, to find the whereabouts of their nests.

Nest searching takes place in the morning and is done only by the female, who has several methods for locating nests. One is to perch fairly high up in trees and watch the activities of the birds around her. Another technique is to move quietly on the ground or low in the shrubs where birds are actively building nests. Still another tactic is to fly noisily into shrubbery to scare the nesting birds and make apparent the location of the nest. The female is constantly on the lookout for new nests so that she can lay three dozen or more eggs over the course of the season.

Brown-headed Cowbirds bring out the ire of many birders because the young Cowbirds hatch sooner, are bigger, eat more and often out-compete other beloved songbird nestlings. However, out of the 40 or so eggs laid by the female, only two or three will hatch offspring that make it to maturity.

Savannah Sparrow nestlings and eggs with a much larger Brown-headed Cowbird nestling. Photo by Kati Fleming via Wikimedia Commons.

Behaviors of Cowbird pairs vary with their territory. In the open spaces of the Midwest, the birds tend not to be terrifically territorial, but more flock oriented and basically promiscuous in their mating. In the woodland areas of the East, the females have fixed territories and seem to be monogamous with a male.

Males compete with each other for dominance using several visual displays. These include the Bill-Tilt, where the bird lifts its head up and tilts its bill toward the sky; the Topple-Over, where the bird fluffs its body feathers, arches its neck and falls forward; and the Head-Forward, where the bird fluffs its body feathers, raises its wings and thrusts its head forward. Their auditory behaviors include the Bubbling song, the Whistle call, the Chatter call and the Cluck call.

As forests were cleared both east and west, the range of the Brown-headed Cowbird increased so that today we can find them in large numbers throughout the United States. While the parasitic nature of these birds has certainly contributed to the decline of our beloved woodland songbirds, so too has habitat fragmentation from development and agriculture.

When it comes to these birds there seem to be more questions than answers. How did these parasitic behaviors evolve? Did Cowbirds ever build their own nests? How do the young recognize their own species? What keeps the young from imprinting on the foster bird parents?

Love them or hate them, these birds are here to stay. They are beautiful to hear and entertaining to watch.

For a close-up look at Brown-headed Cowbirds as well as Swallows and other prairie species, visit Belwin Conservancy’s Bison observation platform.

Members: Join Lynette Anderson on the third Tuesday of each month for a Sunset Prairies Hike at Stagecoach Prairies. Park in the lot off 11th Street South. Our next hike is August 15 from 7:30 p.m. to 9:00 p.m.

Not a member? Join now to enjoy all the benefits of membership and to support our work.