For the Birds Radio Program: Brown-headed Cowbird 2007, Part II
On April 27, 1975, I heard a strangely lovely liquid sound outside my window, and looked out to behold the #16 species on my lifelist, the Brown-headed Cowbird. There were three males on the ground beneath my bird feeder, opening their wings and bowing to a female in their midst. I found them utterly charming and delightful. When I read up about the species, I learned it was a nest parasite, but was comforted to read that baby cowbirds don’t roll eggs and nestlings out of the nest. They do get the lion’s share of food, but if there’s enough food, one or two of the other nestlings in the nest will survive, too. It seemed a cool evolutionary strategy for a species that depended on bison—even if the bison moved on, baby cowbirds could survive by being raised by parent birds who didn’t depend on buffalo.
I lived in Michigan at the time, and soon found out about an endangered bird whose nesting range is unique to that state, the Kirtland’s Warbler. The spread of cowbirds into Michigan beginning in the 1800s had brought this warbler to the brink of extinction—after extensive work, the US Fish and Wildlife Service and the Michigan DNR were hoping against hope to bring the species up to 200 pairs by 1976, and just barely made their goal thanks to several years of extensive cowbird trapping as well as burning to maintain stands of young jackpine trees that Kirtland’s Warblers depend on for nesting.
I concluded that in the case of endangered species like Kirtland’s Warbler, cowbird trapping may be necessary, but really, cowbirds are just trying to make their way in the world and shouldn’t be trapped or harmed where they aren’t endangering whole populations. That’s also the conclusion of a recent monograph about cowbird control.
But when I was down in Oklahoma, I saw another side of cowbird control. In the Wichita Mountains, cowbirds were parasitizing virtually 100% of the nests of another critically endangered species, the Black-capped Vireo. I thought something must be out of kilter, because there were plenty of bison in the Wichitas, so I figured the vireo and cowbird must have evolved together. But historically, the bison left Oklahoma and even Kansas for the northern Great Plains by mid-spring before the vireos returned, because these huge mammals can’t dissipate heat very efficiently. Now of course, no bison remain in America that live the way they did before the pioneers—no way would Americans allow herds of them to wander naturally or cross interstate highways. And Black-capped Vireos are not at all in synch with cowbirds in terms of incubation times—it takes the exceptionally long time of 17 days for eggs of these originally tropical birds to hatch, so cowbirds can have more than a week’s head start on the little vireos, making it almost impossible for even one vireo to survive with a single cowbird egg.
When the Endangered Species Act was passed in the early 1970s, cowbirds were trapped extensively in the Wichita Mountains. Now cowbirds are fairly under control, and it’s turned out to be not only good for the vireos, but for a host of other songbirds. Painted Buntings and Blue-gray Gnatcatchers are more abundant there than anyplace I’ve ever seen in my life. Sure, cowbirds were probably not the #1 reason for these species to be dwindling anywhere, but a population biologist told me the whole issue comes down to a simple principle: species can only hold their own when on average, births match or exceed deaths. We have provided an amazing array of bird killing methods in this world. Manmade killers include communications towers and lighted tall buildings that kill them during their nocturnal flights as well as picture windows, house cats, pesticides, automobiles, power lines, and development in general. The only way neotropical migrant songbirds can withstand the pressures from all these increases in deaths is to increase the number of births—and cowbirds are a critical roadblock to that.
It’s hardly the fault of individual cowbirds that the world has become so skewed. But now that it is, we have some obligations to the songbirds whose world we’ve disrupted to help them in the ways we can, even if those ways involve cowbird control. It’s a sad and difficult issue. But when I see Black-capped Vireos along with bazillions of other songbirds that are far rarer in most places all thriving in a little place in Oklahoma thanks to cowbird control, I think maybe we should be testing whether cowbird control would help other species, such as Wood Thrushes that aren’t endangered yet but are declining dangerously. If we want to continue to have vireos, warblers, thrushes and other neotropical migrants, we’re either going to have to do something to reduce the numbers of bird deaths or do something to increase the numbers of bird births.