Brown-Headed Cowbird
By David Cronenwett
The human tendency to favor certain species of wildlife over others should be a topic of interest for thoughtful conservationists. It is natural for us to admire or identify with particular animals; our Pleistocene ancestors regularly paid homage to them in cave and rock shelter pictographs over the millennia. Various species of bear, lion, mammoth and many others (including at least one owl) are powerfully represented in a manner that can only be regarded as respectful of their spiritual power. This tradition is carried on in the modern age as well; specific organizations work to further the existence of a particular species or class of animals. Birds, elk, mule deer, wild sheep, trout and many others have powerful advocates in their corner and much good work has been done to foster conservation of these creatures and their habitats.
On the other hand, we seem to be equally as passionate about species for which we have no interest or would rather simply do without. Working as an outdoor guide over the years made me acutely aware of this curious human dichotomy. I can remember many occasions where I’d glass a whitetail deer or common bird only to be told that they were hardly worth looking at. Over time, I discovered a great way to playfully rile up a group of hardcore birders was to lead them to a group of brown-headed cowbirds (Molothrus ater). What followed was a predictable stream of gasps, curses and general derision. Though some of this negativity is probably deserved, the story of the cowbird in North America is a complex tale and illustrative of our own problematic relationship with nature.
Cowbirds have a long, evolutionary association with grasslands and buffalo and were historically limited in range by the presence of both. The grazing and hoof disturbance of large animals like bison create ideal feeding conditions for cowbirds, making the insects they prefer easy to locate. This strong relationship to buffalo initiated some fascinating cowbird adaptations with the most notable (some might say notorious) being its nest-parasitism behavior. Because bison continually roamed the Great Plains, cowbirds were unable to stay in one place long enough to nest and raise young. An elegant solution to this problem was to identify a suitable host nest, deposit an egg and let the unsuspecting host raise the cowbird fledgling as its own.
This reproductive strategy requires no nest building, but female cowbirds generally attempt to lay a single egg per host-nest over a large area, translating to over thirty eggs per season…a huge number. This means cowbirds need an unusually large amount of calcium in their diet, often acquired by consuming the egg shells of other birds. These reproductive adaptations have proven highly successful for Brown-headed Cowbirds, but they also suppress other native bird populations in some circumstances. Cowbird eggs only require a ten to eleven day incubation period whereas many host-eggs need a day or two longer. Early hatching and very aggressive behavior toward its nest-mates (including smothering or ejecting) allow cowbird young to out- compete host fledglings. Most host species do not recognize that they have been duped, though some do. Yellow warblers will often build another nest on top of the original one, rather than raise a cowbird in their midst. Gray catbirds that find a cowbird egg in their own nest will destroy or reject it 95% of the time. In Montana, 53 species of birds are known to be used by cowbirds as brood-hosts. Because they are native to the plains, they exist here as just another player in the ecological system. However, these birds can cause real problems where they have more recently appeared.
The creation of “edge” habitats as a result of clearing forests, and the expansion of agriculture and livestock in landscapes across North America have allowed cowbirds to colonize new areas, negatively affecting some native birds who have no coping mechanisms to deal with such a formidable competitor. Rare birds like Kirtland’s Warbler and Black-capped Vireo are known to be especially vulnerable to the presence of cowbirds. The upper Midwest, Northeast and Southeast parts of the country now support robust populations of this western grassland bird. By all accounts, cowbirds are an incredible survivor who have benefitted tremendously by the presence of people. Brown-headed cowbirds are protected by the Migratory Bird Act, but may be lethally managed under special permit to specifically protect sensitive songbirds in their range.
The Brown-headed Cowbird, like all species that end up out of place, must be judged on a case by case basis. In their native environment, various factors keep their population in check with other birds. Where we’ve created conditions allowing them to displace resident birds in new landscapes, it is our responsibility to manage their impact on the natives. Some people feel that any sort of nature-management, whether cowbirds or knapweed, to be evidence of a certain human arrogance, but I find this idea shortsighted and uninformed. If you see cowbirds in Montana during the upcoming breeding season, maybe even in the act of parasitizing a warbler nest, remember their unique natural history and adaptations to bison and grass. It is also a good time to consider the potential consequences of disturbing ecosystems beyond what they are historically accustomed to.