Focal Species Close-up: Brown-headed Cowbirds
The Brown-headed Cowbird, a much-maligned species, has been dubbed a “social outcast,”* a “feathered wretch,”ª and is often considered to be an immoral and slothful pest whose villainy is responsible for declining of songbird populations. Frank Chapman, the self-taught ornithologist and curator of ornithology at the American Museum of Natural History for several decades, described the sounds cowbird males make in spring as “guttural bubblings produced with apparently nauseous effort,”˚ and continued to deplore the “despicable” nature of cowbirds and their nest-parasitism. While we might expect the man who first proposed what is now known as the Christmas Bird Count as a replacement for the Christmas “Side Hunt” (which involved guns instead of pencils to keep score) to show a little more even-handedness towards bird species (I wonder what he said about vultures), the disdain for cowbirds is understandable.
As nest parasites, Brown-headed Cowbird females lay their thick-shelled eggs in the nests of other species and let them take care of the offspring. The cowbird hatchlings generally grow faster and larger than the host chicks and sometimes even outsize the host parent, drawing resources from the hungry host family. This behavior is certainly morally dubious from a human point of view, and it is true that many bird species will attack Brown-headed Cowbirds on sight, but this is no reason to hate the cowbird for its fascinating life-history adaptations to breeding biology. This post isn’t the place to go too far in depth, so I’ll share just a couple cool elements of the parasitic species’ natural history that hopefully show the Brown-headed Cowbird is an interesting and valuable subject of study. I will write assuming that my reader is at least slightly familiar with the general features of the bird from the Cornell Lab of Ornithology’s relevant pages at All About Birds, Birds of North America Online, and the Celebrate Urban Birds quick guide.
Let’s start with the nest. Brown-headed Cowbirds don’t make one. Instead, the female has to find an appropriate nest built by another species. They can do this by silently watching potential host activity, searching for nests systematically in silence, or noisily searching and flushing out hosts. They must have several nests under observation so they can lay eggs at daily intervals. A female cowbird’s average laying time is 41 seconds, compared to most songbirds that range from 20.7 to 103 minutes†. Timing is key because if a female lays too soon, her host parents might desert the nest or build a new nest floor on top of the egg (Yellow Warblers often do this); too late, and the cowbird offspring will be at a disadvantage with their foster siblings. The female cowbird needs to get in, lay the egg, and get out before the parasitized parents come home to see her at work. Blue Jays, Eastern Kingbirds, Gray Catbirds, American Robins, and some other species targeted by cowbirds remove the parasite eggs from their nests if they recognize them.
But what if a host doesn’t recognize the new eggs as foreign goods? Some ornithologists think that perhaps part of cowbirds’ success at not having their eggs thrown out or broken is that many of the host species are easily affected by supernormal stimuli, such as larger eggs, more boldly spotted eggs, or simply more eggs. Put simply, an egg with supernormal stimulus is one that the host bird might find instinctively attractive: wouldn’t you be proud to have an extraordinarily large egg, or more eggs than your neighbor? Once the cowbird hatches, it has a further superstimulus in its favor: intensely loud begging vocalization, which almost guarantees feeding by the host parents even once the cowbird chick is double their size, like in the photo above.
The lack of host discrimination against dissimilar eggs and hatchlings hints that cowbirds have relatively recently begun parasitizing several eastern species like the Wood Thrush or Red-eyed Vireo, almost certainly as a result of deforestation and expansion of cattle, sheep, and horse—and thus cowbird—habitat. So in part, we might blame any decline in songbird populations on our purposeful ecosystem change and accidental expansion of perfect habitat for the cowbirds. Although cowbirds are not as territorial as a bird with its own nest might be, it does seem that females defend ranges of territory in which host nests are deemed abundant or of high quality. It would make sense for a female cowbird to try to exclude other nest parasites from her area because multiple parasitism of a nest would reduce reproductive success of all eggs in the nest. Relatively low territoriality, especially in the males, seems to explain the lack of a true song in the Brown-headed Cowbird.
As habitat fragmentation further encourages the spread of Brown-headed Cowbirds, we will continue to see songbird populations change, and it will be interesting to see if any behavioral adaptations are able to occur in the evolutionary short-term. If you know of any nests in your area, keep an eye out for one or two abnormal eggs appearing alongside the host eggs, and watch what happens! Cowbird chicks can make even the most mundane nest a lot more funky.
* Pearson et al., Birds of North Carolina, 1959
ª Pearson and Burroughs, Birds of North America,1917
˚ Chapman, Birds of Eastern North America, 1902
† Sealy, S.G., D.L. Neudorf, & D.P. Hill. 1995. “Rapid laying by Brown-headed Cowbirds Molothrus ater and other parasitic birds,” Ibis 137:76-84.
Other sources and further reading:
Jackson, N.H. & D.D. Roby. 1992. “Fecundity and egg-laying patterns of captive yearling Brown-headed Cowbirds,” Condor 94:585-589.
Ortega, Catherine. Cowbirds and other brood parasites, 1998.
Skutch, Alexander. Orioles, blackbirds, and their kin: a natural history, 1996.
Woodward, P. W. 1983. “Behavioral ecology of fledgling Brown-headed Cowbirds and their hosts,” Condor 85:151-163.