Several years ago I brought a fledgling home from the Wildlife Center and put it in a cage with a juvenile house finch. While they resembled each other, the new bird was larger, heavier and streaked differently. Later I heard the newcomer piping, just like the young cardinal I was raising in the adjacent cage. When my husband saw it, he agreed that although the new bird looked like the finch but sounded like a cardinal, he said without hesitation, “It’s a cowbird that was raised by a cardinal.”
The brown-headed cowbird (Molothrus ater) is a member of the blackbird family. Males have a metallic green-black body and females have a gray-brown head and pale brown body. Both are about 7 ½” long at maturity.
Widely distributed over woodlands, farms, cities and ranches, the ubiquitous cowbird has a dark side. An indigenous species that evolved in the grasslands of North America, it’s one of the ‘brood-parasitic’ cowbird species. Brood parasites lay their eggs in the nests of other birds, known as hosts, and then let the hosts incubate, feed and raise their young.
This species was first named “cowpen bird” by an 18th century naturalist because of its fondness for feeding in cattle pens. Later they were called buffalo birds because they followed the herds and fed on insects on and around the big animals as they roamed the Plains. The colonization and cultivation of most of North America by early settlers meant an end to the large buffalo population, but not the end for the buffalo bird: they simply turned their attention to domesticated cattle, and were soon dubbed cowbirds.
Since cowbirds were also always on the move, one theory contends that they didn’t have time to build nests or return constantly with food for their offspring, so they laid their eggs in the nests of other birds.
An essay by George Tobin remarks, “The single-parent lifestyle has never been a popular option among bird species. The vast majority of avian species have some form of two-parent support system, [but] in the end, the cowbird female made the best of a bad situation by leaving her eggs in the nests of nearby birds of another species. She then flew off to rejoin the others back at the herd.”
Although a cowbird usually lays only a single egg per nest of a host bird, it’s not unusual for more than one cowbird to lay an egg in the same host’s nest. Brown-headed cowbirds lay their eggs in the nests of more than 150 species of birds, and some of the most common are warblers, vireos, flycatchers and finches. The brown-headed cowbird’s unusual habits pose a threat to many species of songbirds in different geographic areas of Texas and the U.S., and foster arguments among experts on how to best solve the problem. Cowbird trapping has been implemented in a number of areas, including in Texas.
The female cowbird watches for nest-building activity of a host bird and then waits for her to lay her eggs. Usually just before dawn the host bird leaves its nest and the cowbird then deposits one of her eggs and removes one of the host’s eggs from the nest.
Some of the birds whose nests the cowbird invades have learned to distinguish the cowbird’s egg and the host removes the egg or deserts the nest. Some birds build a new nest over the top of the one being occupied by the cowbird’s egg.
If the cowbird egg isn’t recognized, it hatches before the host bird’s eggs. Once hatched, the cowbird youngster noisily and persistently begs for food and grabs much of it that’s brought to the nest by the victimized parents. Sometimes weaker host offspring starve, suffocate, or are pushed out of the nest as the little intruder gradually takes over the entire space. The cowbird chick develops rapidly and often grows bigger than the host parent. The chick remains dependent on the “parents” about two weeks after leaving the nest.
During the winter in our area there are sometimes large flocks of cowbirds that roost noisily in trees. They come in just before dusk in large, undulating flocks to settle for the night.
The brown-headed cowbird has been extensively studied but some mysteries remain: why does it not completely imprint on the host, but instead quickly leaves its foster parents to join the cowbird flock? In other words, how does it know it's a cowbird? Some research even suggests the female cowbird seeks out a nest of the same species that raised her to lay her own eggs. Perhaps that’s why the cowbird I raised one year was so much like the cardinal in song and behavior.
You can see the McCaslands’ photos of the cardinal feeding the cowbird below and for a good close-up view of both brown-headed cowbird sexes visit http://www.holoweb.com/cannon/broffwn.htm . Click on any of the thumbnails below to view the images.
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