Blane Klemek: The brown-headed cowbird is a survivor on the Great Plains
A couple of weeks ago I heard the familiar and fascinating songs of courting male brown-headed cowbirds. The songs — if one can call them songs — are nearly indescribable, but can easily be portrayed as incredibly complex and interesting to listen to.
After spotting the source of the songs and calls, I observed the typical cowbird spectacle that I came to know and appreciate during my summers working on wildlife research projects on the Great Plains of North Dakota.
In this case there were three males jockeying for positions on a single tree limb in what appeared to be vain attempts at wooing a lone female. Each male sang and postured — pointing their beaks skyward and perching rigidly. Soon afterwards, the entire group departed to another nearby perch to resume the serenade.
As its name implies, cowbirds are often associated with cattle and the male of the species has a brown head. It is a member of the same family (Icteridae) that blackbirds, orioles, bobolinks, and meadowlarks belong to. No other bird in the family, however, has a more interesting nesting strategy.
Cowbirds are known by ornithologists as “brood or nest parasites.” Brood parasitism is defined as “…the surreptitious addition of eggs to another female’s nest [and] is a common form of cheating.” In the avian world the cowbird is not the only bird that lays its eggs in other birds’ nests. In fact, while not considered an obligate brood parasite (those birds that never build their own nests or raise their own young), wood duck hens often lay eggs in other female wood ducks’ nests.
Brown-headed cowbirds are obligate brood parasites and the behavior has evolved independently at least seven times among birds. What brood parasitism allows these birds to avoid are the high costs and risks of incubating and caring for offspring. How and why it evolved are interesting questions to ponder. It has been theorized that brood parasitism in cowbirds evolved in response to the nomadic ways of bison that once roamed this continent.
Cowbirds followed herds of grazing bison and fed on insects that the beasts would disturb as they moved through the grasses. The birds would also alight on the animals’ backs and heads to pick insects from their hide and hair.
The great herds of buffalo never spent much time in one place; they were always on the move. So, if the brown-headed cowbird was to survive as a species while continuing to follow the herds for food, the birds had to come up with a timely method to raise its young. The solution? Let other birds do the job.
Consequently, in order to keep up with the bison, adult female cowbirds would simply seek out the nests of other species of birds, lay an egg, and move on. The hosts therefore became, in every sense of the word, foster parents to the fast-growing cowbird chicks.
Brown-headed cowbirds can lay from 30 to 40 eggs per season in weekly sets of two to five eggs. Usually, only one egg is laid in a host nest. To reduce the chance of host recognition and rejection, cowbird eggs even resemble the hosts’ eggs. Host nests can be from a number of different species. The nests of flycatchers, sparrows, warblers, and vireos are commonly parasitized.
A single, deposited egg has the advantage from the start. The eggs of brood parasites normally hatch two to four days sooner than do those of the host. Furthermore, the cowbird hatchling grows at a much faster rate than its foster nest-mates do. Often is the case that the cowbird hatchling out-competes its nestlings, receives most of the parental care, and thus, the lion’s share of the food. The cowbird chick frequently becomes the only hatchling to fledge from its foster parents’ nest.
During my summers conducting breeding bird surveys in North Dakota, I observed and recorded the activities and behaviors of many species of birds. I became especially interested in the mating behaviors of cowbirds. As previously eluded to, male cowbirds produce one of the most varied and complex of all bird songs. It is written that the short and unusual “glug-glug-glee” song encompasses the greatest frequency range found in any single bird song. Females choose males that can best perform such vocalizations.
Indeed, regardless of how one views the brown-headed cowbird, the bird and its species is a survivor — and they do so in a most extraordinary manner. While the species nesting behavior comes at the expense of other species of birds, research seems to indicate that we humans are partly to blame.
As we continue to create more edge habitat through woodland fragmentation and urban sprawl, brown-headed cowbirds are more easily able to parasitize forest birds’ nests. Facts aside, these remarkable birds have been around a lot longer than we have and they will continue to thrive for us to observe as we get out and enjoy the great outdoors.
(Klemek is a DNR Wildlife Manager. You can contact him at firstname.lastname@example.org.)