Catching cowbirds with the Reach kids
Recently, I helped the West Fargo School Districts Reach program with their Prairie Day outdoor learning at various work stations on the Henke farmstead. My station was bird-watching. The kids were a joy to work with and displayed enthusiasm and knowledge I hadnt expected. (Jolene Beckman-Sternhagen and Kim Sandvik should be patted on the back for what they accomplish with our kids). What made it fun for my groups was an informal contest to see which could find the most bird species. During the few hours spent there, the dominant bird seemed to be the Brown-headed Cowbird (Molothrus ater).
Cowbirds are easily found around here and interesting creatures in a lot of ways. They are relatively small black birdsthough not technically a Blackbirdwith short, black, conical bills and black eyes. Males are uniformly glossy black with a brown head and neck. Females are a more muted, brownish-gray. Partially migratory, the birds winter in the southern U.S., often forming large mixed flocks with Red-winged Blackbirds, Common Grackles, and Rusty Blackbirds.
Before settlement this bird, once called the buffalo bird, was common only on the Great Plains. Cowbirds would associate with vast herds of bison and feed on insects kicked up by the grazing animals. In order to continue this habit of following the herds, cowbirds developed an unusual nesting strategy: They began laying eggs in the nests of other birds; a behavior scientists call brood parasitism. In fact the bird builds no nest itself. Instead, cowbirds rely entirely on surrogate hosts to incubate and raise their young.
A typical female cowbird will lay about 40 eggs per year in various nests. The egg usually hatches slightly before the hosts own eggs. Given the rapid growth of cowbird chicks, the hosts own nestlings are often abandoned or ignored. Over 220 species have been identified as victims of cowbird brood parasitism including Blue-winged Teal and Red-headed Woodpecker along with many of our common songbirds. Many bird species, however, evolved with cowbirds and developed defensive strategies. Some are able to recognize the intruders egg and either dispose of it or re-nest altogether.
With expansion of settlement in the 18th and 19th centuries came fragmentation of forested areas. As a result, cowbirds extended their range exponentially to the point where they now nest across virtually the entire continent south of the Arctic. Instead of bison, cowbirds now associate with almost any grazing animals such as cattle, sheep or goats.
With the impact of cowbird parasitism not fully understood, there is considerable debate regarding human controls on the birds. Some experts consider cowbirds a major threat to songbird populations. Many studies are still taking place and at least one attributes no significance to cowbird parasitism in the decline of host bird numbers. Another study claims cowbird populations themselves are actually in decline. It seems a difficult problem to answer considering the continental scale of it. Certainly, on a local level, parasitism can have a deleterious impact on some species however.
While the problem is controversial, most agree some sort of control is needed in the local nesting areas of at-risk endangered species. Birds like the Kirtlands Warbler (Michigan) and the Black-capped Vireo (Texas) are just two examples. Lethal measures taken in these zones include trapping, shooting and the removal of cowbird eggs. Trapping seems to be the most efficient tool for removing large numbers of cowbirds.
Success is measured in numbers: On a military reservation in Texas, cowbird brood parasitism against the Black-capped Vireo went from 90% in 1987 to just 22% by 1996. Still, scientists recognize the obvious limitations of these measures (including cost) and caution they can only be used in small areas to boost local host populations. More critical a problem than cowbirds, they say, is habitat loss.
The next time you hear a squeaky, gurgling whistle similar to a rusty hinge, you may be hearing a Brown-headed Cowbird. Undoubtedly, the birds are looking for a nest to parasitize this time of year. But to cast judgment on this wild bird on the basis of certain behaviors is probably beyond our understanding. And likely an issue best left for nature to resolve itself.