Brown-headed cowbirds have become so abundant in Illinois that they are threatening to eliminate our state’s nesting populations of wood thrushes, hooded warblers, ovenbirds, and scarlet tanagers, among other species. That is the provisional conclusion arrived at by Scott Robinson of the Illinois Natural History Survey after completing the first year of a projected five-year study of the birds of the woodlots and woodlands of central and southern Illinois.
They were also restricted by their habitat requirements. They are primarily edge birds, at home on the borders between the open grasslands where they feed and the trees they use for perching, singing, and surveying the surroundings in search of nests. You would be unlikely to find them on the open prairie or in the heart of a large forest.
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Perhaps the hardest-hit species is the wood thrush, a relative of the robin that feeds on the forest floor and nests in low shrubs. Studies made in central Wisconsin, central Illinois, and southern Illinois revealed that more than 90 percent of wood-thrush nests contained cowbird eggs. In Illinois a typical wood-thrush nest contains more cowbird than wood-thrush eggs. “I don’t think any population can withstand that kind of parasitism,” Robinson says.
At this point in his study, Robinson is not ready to make any definite recommendations on how we might manage our forests to protect these birds. However, he is willing to say that bigger is better, and he, along with many others, is urging the U.S. Forest Service to set aside the 20 percent of the Shawnee that contains large forest tracts. He hopes by the end of the study to be able to make recommendations for management practices that would make even our smaller woodlands hospitable to forest birds and less hospitable to cowbirds.
