WILD WNY / Despised but interesting?
Consider the cowbird
Cowbirds developed nomadic habits when the species used to follow herds of buffalo.
Most of us have our favorite birds—colorful bluebirds, cardinals, or scarlet tanagers, or familiar song sparrows, robins, or house wrens. We have most disliked birds as well. The cormorant that destroys its nesting area with its guano is one. The cowbird is another.
Why hate the cowbird? Some anthropomorphize cowbirds as cheaters, because they don’t build their own nests. Instead, the females lay their eggs in the nests of other songbirds, not only turning over care of their young to those birds but, in the process, usually starving the other species’ own offspring. The cowbird chick is larger and more aggressive and takes the lion’s share of the food its stepparents deliver to the nest. It also hatches quicker, thus gaining a head start on the natural chicks. Some researchers are so upset by this that they describe cowbirds as evil.
It is believed that this cowbird behavior developed when much of our nation was populated with huge herds of buffalo. The nomadic herds depleted the vegetation of an area before moving on. Cowbirds followed the buffalo and thus had to become nomadic as well, which meant they could not stop to raise their own families. The solution: brood parasitic behavior.
In fairness to the cowbird, I point out that it is not the only species that practices this technique. The sweet-singing European cuckoo is a brood parasite as well. (Our North American cuckoos are not, however.) And many of our cavity-nesting ducks like wood ducks, hooded mergansers, and goldeneyes are guilty of what is also called egg-dumping. Nest boxes, when checked, sometimes turn up with several dozen eggs. Even if the original nesting female does not abandon the nest, the heat of her body will only be enough to hatch a few of the eggs. There are many brood parasites: worldwide, over 200 species have adopted this kind of behavior.
Our brown-headed cowbird is an icterid, that is, a member of the bird family that includes blackbirds, meadowlarks, and orioles. It is smaller than the familiar red-winged blackbird and less strikingly colored. The male cowbird has a brown head and the rest of its body is shiny black. Female cowbirds are uniformly gray. Their song, if you can call it that, is a thin gurgling whistle. You can often see the males in spring courting females; they hunch up their wings as they squeak out their passion.
Indigo bunting nest with brown-headed cowbird egg
What research tells us about cowbirds
How do cowbirds pick nests to parasitize? Clearly, they are opportunistic: over 220 species have been recorded as “hosts.” Close observation indicates that female cowbirds watch other species as they build their nests and begin to lay, then slip in when the host birds are away to deposit their own eggs. Some evidence suggests that, with several choices of species available, the cowbird will pick the smallest, thus characterizing itself as a bully as well as a sneak. It has also been shown that individual cowbirds remember a species they have successfully parasitized from one season to the next, and choose that same species in following years.
You might think that appropriate behavior for any bird mother would be to throw out the new and significantly larger egg. And, indeed, some species do this: the brown thrasher is one. But most birds apparently do not identify the added egg as different and, in the case of smaller species, are unable to heave the larger eggs out of the nests. One species faced with this problem, the yellow warbler, simply builds more nest over the cowbird’s and other earlier eggs and lays another clutch. Sometimes, if the cowbird returns, the process is repeated until finally a multistory nest results with levels of infertile eggs buried in it.
It turns out that cowbirds are undeterred by birds who defend against them by removing the cowbird eggs. They often return to lay more eggs, which researchers call farming. They are also guilty of what has been termed mafia-type behavior, revisiting defended nests to destroy the rejecting host’s remaining eggs. Worse behavior, still!
Birds do not inherit their songs. They learn their species’ songs from their parents, so you might think that cowbirds would learn the song of the species that raises them. That they do not has been the subject of several studies. One suggests that their brain growth in the area where song-learning takes place develops differently in cowbirds; a kind of password is required before their song is learned. Another found that young cowbirds leave their nest at night before they are independent of their hosts to spend time with other young cowbirds, just as human teenagers do in what we call street-corner behavior. They evidently share their squawks there.
In this column, I have called on a number of human characteristics to compare with those of the cowbird. This is, of course, not fair to the birds. We have choice in our behavior: we can choose to be sneaky, to bully those weaker than us, and even to adopt mafia-like behavior. Birds don’t have this choice; their lives are controlled by their locked-in brain characteristics and evolutionary history. We can no sooner blame cowbirds than we can the hawk that flashes in to snatch a nuthatch from our feeder.
Of course, that doesn’t mean that we are required to like them.