In the Shawnee National Forest the red-eyed vireos are getting scarcer, the cerulean warblers are nearly extirpated, and the remaining wood thrushes are raising cowbirds.
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And what the data reveal is a picture of the lack of reproductive success of many species of neotropical migrant forest-interior birds. These birds are hurting in part because of the loss of winter habitat caused by the destruction of tropical forests, but their problems on their summer range seem to stem from the fragmentation of the local landscape, which exposes these birds to the hazards of life on the forest edge, particularly to high rates of predation–by raccoons, opossums, and other animals–and brood parasitism by the brown-headed cowbird. Nothing in the evolutionary histories of the forest-interior birds has prepared them to face such hazards. They lack the defensive strategies of such edge species as the gray catbird–which removes cowbird eggs from its nest–and their long migrations shorten their breeding season so that they cannot adopt the strategy of song sparrows and cardinals, which may nest as many as five times each summer.
The Shawnee project grew out of a study Robinson began in 1985 at Lake Shelbyville, a large artificial lake south of Decatur in central Illinois. Robinson describes the landscape around the lake as “hacked up beyond belief.” It is cornfields and houses and scattered woodlots, none of them of any size. Wood thrushes were nesting in the woodlots, but when he surveyed their nests he found parasitism rates almost beyond belief: as many as 11 cowbird eggs in one nest, and many nests with nothing but cowbird eggs.
Not all species were equally affected by the situation. While wood thrushes seemed to be heading for a catastrophe, worm-eating warblers–a species that studies in the east had shown to be extremely sensitive to landscape fragmentation–were doing rather well. So were Kentucky warblers, although neither they nor the worm-eating warblers were as successful as other studies had shown them to be in large tracts. The study had begun as a way of measuring edge effects and finding ways to minimize them, but the surprises in the data were raising complicated questions about what species were doing well and what kinds of places they did well in.
When he began to study the Shawnee, Robinson expected to find that agricultural openings would be worse than clear-cuttings. He thought cowbirds would stick close to pastures and raccoons and opossums would be near cornfields, but he discovered that he was wrong. “We are absolutely full with all those animals, especially the cowbird. They are just at saturation level.”
Further details will have to await a more extended analysis of the data and what Robinson hopes will be two more years of study. “A five-year study will level out some of the year-to-year variations and give us a clearer picture,” he says. His fear is that the picture will be a glimpse of the future everywhere in eastern North America as habitat continues to be fragmented.
