Blane Klemek column: Cowbirds possess unusual song, nesting behavior
I often try describing the songs and calls of birds by writing what I and other writers and ornithologists believe they sound like.
For example, by writing the "fee-bee" call of the black-capped chickadee, or the "Old Sam Peabody" song of the white-throated sparrow, or the "onk-a-ronk" vocalization of the American bittern, the reader might become better acquainted with the bird in question.
Offering written descriptions of bird vocalizations is difficult to do, harder still to imagine I suppose, but such depictions sometimes get the job done - that is, giving the readers an idea of what something might sound like while providing them a stronger basis of understanding for when they encounter the bird in the field.
Yet, I do not believe there's a person alive who can accurately describe by the written word, much less by voice mimicry, the vocalizations of the brown-headed cowbird, especially the singing male. One account that I've seen written barely attempts to describe the male cowbird's complex song by simply stating that it's a "gurgling whistle."
Indeed, if one can get past the fact that the brown-headed cowbird is a "brood parasite," a phrase that conjures negativity, then one can begin to appreciate the natural history of this highly interesting and native bird for what the species is all about.
As their name implies, cowbirds are often associated with cattle. As well, the male of the species has a brown head. It is a member of the same family that blackbirds, orioles, bobolinks and meadowlarks belong to. No other bird in the family, however, has a more interesting nesting strategy.
During my summers conducting breeding bird surveys on the plains of 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. Male cowbirds produce one of the most varied and complex of all bird songs. 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.
As I already mentioned, cowbirds are known 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, at least among cowbirds, that brood parasitism 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 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 a cowbird chick.
Brown-headed cowbirds can lay from 30 to 40 eggs per season in weekly sets of two-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-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.
Regardless of how one views the brown-headed cowbird, the birds are simply trying to survive. 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, development and urban sprawl, brown-headed cowbirds are more easily able to parasitize forest birds' nests.
Still, these remarkable birds have been around a long, long time and they will continue to thrive for us to observe as we get out and enjoy the great outdoors.
Blane Klemek is the Bemidji area assistant wildlife manager, DNR Division of Fish & Wildlife. He can be reached at firstname.lastname@example.org.