Everywhere I go here in central Illinois, one of the easy-peasy species I'm bound to see is the brown-headed cowbird. Often, I hear it first--giving a short, bubbling, burbling song that I've heard described as it's "water call." At Sugar Grove Nature Center over the winter, I would frequently see flocks of up to 100 cowbirds, hanging out around the feeders.
The male cowbird is not unattractive: a shiny black body with a glossy brown head. Their behavior, however, is notorious: like the more famous European cuckoo, the cowbird is a brood parasite, recruiting other species as unwitting nannies by depositing their eggs in other bird's nests. In the days of Yore, this wasn't really a problem: cowbirds are grassland birds, who traditionally followed the bison. Since following bison and incubating eggs aren't really compatible, they developed their habit of foisting their young on others.
Now that humans have fragmented forest habitat, creating a lot more open and edge environments, the cowbirds are flourishing, even though the bison are gone. I love birds, but seeing so many cowbirds bugs me: I can't help but think of all the unhatched songbirds they represent, and they have become kind of symbolic for me, of what we humans have done wrong. I mean, it's not the cowbirds fault.
Today I decided to look up cowbirds on the Internet and see just how bad they are. I learned some interesting cowbird factoids: for example, that individual female cowbirds will have preferences for parasitizing certain hosts. Cowbirds have parasitized over 200 different species, so they can't be called choosy birds, although sometimes they make bad decisions. House finches feed their young diets that baby cowbirds cannot thrive on, for example, so young birds raised by house finches rarely survive. Some birds get savvy to the cowbird's ways, and have different defenses against them: yellow warblers will build a new nest over the top of a nest with a cowbird egg, gnatcatchers will abandon their nests, and brown thrashers will destroy the cowbird eggs. Most species, however, are oblivious to the fact that their young have been switched at birth, so to speak, and will happily raise the young cowbird as their own. Even those species who have caught on are not safe: the "cowbird mafia" will actually check the nests they have left eggs in, and retaliate if their egg is gone. For all of these reasons, cowbirds have a bad reputation, and have been blamed for declining songbird populations.
With all these cowbird prejudices in mind, I was surprised to read on the Audubon website that, with the exception of a few endangered species, such as the Kirtland's warbler and the black-capped vireo, cowbirds are not largely to blame for songbird declines (sadly, the blame would lie, once again, with us humans). In fact, cowbird populations are even declining slightly, and most species who unwittingly raise a cowbird can go on to re-nest later in the year. Even species that are heavily parasitized by cowbirds, such as song sparrows, only lose a small percentage of their chicks because of them.
To control cowbirds (again, information is from the Audubon website), wildlife managers rely on methods such as trapping them, removing their eggs from nests, and shooting them at their roosting sites. (In Luke Dempsey's book "A Supremely Bad Idea," he describes how up to 4,000 cowbirds a year are trapped and then squeezed to death to protect the endangered Kirkland's warbler.) However, with the exception of cowbird murder to protect endangered species (sorry if my pro-animal rights bias is creeping out), these actions are controversial--for one thing, they are expensive and time-consuming, they don't necessarily improve nesting success for other birds, and they are merely a stop-gap measure that does not address the underlying problem: the reason that cowbirds are able to do this is because of the changes in habitat that we are creating. Until we restore woodland habitats, we are doing nothing to solve the real problem.
Last summer, I felt privileged to witness a couple of real-life cowbird moments. One was at the McKee Marsh in the Blackwell Forest Preserve (by Warrenville, IL), where a pair of blue-gray gnatcatchers had caught a pair of cowbirds skulking around their nests. The gnatcatchers were clearly upset over this, jumping and diving at the female cowbird until she flew away (egg laid or not? That I could not tell.) The male was observing all this from a branch higher up in the tree. Meanwhile, the gnatchatchers were so upset that they attacked an unwitting white-breasted nuthatch who had the bad luck to land on that tree trunk just then.
The second instance was at the Morton Arboretum, where I saw a cowbird chick being fed by its adoptive parent, a song sparrow. The cowbird looked so different from its "parent" -- for one thing, it was huge, much bigger than the little sparrow ferrying insects to its hungry beak -- that it was hard to believe that the song sparrow hadn't caught on. Well, who's to say it hadn't? I don't know what birds are thinking. The song sparrow left, after depositing the meal it the cowbird's beak, and then the baby cowbird gave a tiny chirp and settled down on its branch in the sunlight to rest. Honestly, that little chick is one of the "cutest" things I've ever seen....
Kind of hard to hate cowbirds after that. (After all, I don't hate humans, and it's really our fault about the habitat...)