By Gayle Pille
Special to NKyTribune
Cowbirds have an interesting history. The handsome Brown-headed Cowbird that is so common in our backyards today is originally a bird of the Great Plains. Cowbirds co-evolved with large bison herds of the prairie, foraging on insects and prairie grass seeds kicked up by the bison as they made their seasonal migrations north and south. In fact early settlers called them “buffalo birds.”
Since bison were always on the move, it was necessary for buffalo birds to be on the move as well. During migration, bison would travel up to 400 miles. The nomadic lifestyles of bison and buffalo birds left the birds no time for nest building and parenting.
The solution for these avian nomads of the Great Plains was to lay their eggs in the nests of other birds and let unassuming foster parents raise their chicks. The buffalo birds could then continue their wander-lust and follow the bison herds they were so dependent upon for their livelihood.
When bison were slaughtered by early European settlers, one might think it was also the end of buffalo birds. The birds though adapted to new ungulates, domestic cattle, and became known as “cowbirds.” They then expanded their range to the continental United States, Alaska, and southern Canada.
Brown-headed Cowbirds are North America’s only brood parasites. They don’t build their own nests or raise their own young. They lay their eggs in the nests of other birds, which in turn raise the young cowbirds as their own. They are members of the blackbird (Icteridae) family, which includes grackles, orioles, meadowlarks, and bobolinks. A native songbird, fossil records show they have been in North America for at least a half million years.
As brood parasites, cowbirds have been known to parasitize more than 220 different bird species; while more than 150 different host species have been known to rear cowbird chicks. Host species range from vireos and warblers to cardinals and thrushes. A single female cowbird can lay 40 to 80 eggs during the eight-week breeding season.
Cowbirds will stake out a territory and monitor it for potential host nests, especially during the morning hours. When the opportunity presents itself, the female cowbird will lay an egg in an unsuspecting host’s nest, often times removing one of the eggs belonging to the host bird. She’ll do this daily for more than a month and may lay several of her own eggs in the nest of a host bird.
Cowbird eggs are often larger, and hatch a day or two earlier than other birds. This gives them a decisive head start in life compared to their “adopted” siblings. They are usually the larger and more aggressive youngsters in the nest and out-compete the others for food and space. When it’s time to fledge the nest, it’s not uncommon for cowbirds to be the only surviving chicks.
Some host birds have developed defense mechanisms against the cowbird’s deleterious habits. Birds most likely to recognize and reject cowbird eggs are birds that co-evolved with them on the prairie, including red-winged blackbirds, grackles, blue jays, robins, cedar waxwings, thrashers, and orioles.
Woodland birds, as warblers and vireos, have no such immunity against cowbirds and pay a heavy price when their nests are parasitized.
Ornithologist Kevin Eckerle, formerly of Edgewood, has studied and is a proponent of cowbirds. “Cowbirds are only taking advantage of what humans have done by cutting up land into parcels, exposing the inside of the forest to cowbirds,” said Kevin. “These are changes in the landscape that humans have brought about and serve cowbirds well.”
Cowbirds rarely occur in large, untouched forests. They are creatures of open spaces and don’t like deep woods. Forest fragmentation has opened pathways for cowbirds to find new sources of host species.
“Cowbirds are doing exactly what they’ve evolved to do,” said Kevin. They’re taking advantage of habitats we’ve provided for them. The way to control cowbirds is to keep the forest as large and intact as possible.”
There are those who loathe cowbirds for their unusual breeding habits and the impact they have on other bird species; but buffalo birds didn’t extirpate bison herds and forever change the landscape. We did.
Gayle Pille is a local naturalist and nature writer who many know through her work to establish the five-mile network of nature trails at Highland Cemetery in Ft. Mitchell. She created the cemetery’s popular 25-year-old Wildlife Enhancement Program and works with a small team of volunteers to maintain the cemetery’s wooded walking paths. An avid birdwatcher, Gayle also builds custom wildlife nest boxes for businesses, parks and residences through her business, www.woodlandhabitat.com
Contact her at gaylepille@yahoo.com
Featured photo by Harry Neiman