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.

Brown-headed cowbirds are brood parasites. They lay their eggs in the nests of other birds and leave the job of rearing their young to the unwitting foster parents. On average, you can figure that each cowbird raised means at least one less fledgling produced by the parasitized nest.

Cowbirds are native to North America. They are insect-eating birds whose common name comes from their habit of following herds of cattle and eating the insects stirred up by the herds’ passing. In presettlement North America they followed buffalo or elk herds. Some ornithologists have speculated that their parasitic habits developed out of their nomadic life. They could not stop for a month to rear a brood, because the ungulates they depended on were always on the move.

In the past their parasitism meant the death of individuals, but it didn’t pose a threat to any species. Their small numbers and nomadic habits spread out and reduced the impact of their mode of life.

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.

The other edge birds developed defenses against them. Catbirds, for example, recognize cowbird eggs and remove them from their nests. Yellow warblers build a new nest on top of a parasitized nest and lay a new clutch of eggs. But the birds of the forest interior never developed such defenses because they rarely encountered cowbirds.

That happy situation began to change when settlers arrived. We began to plant trees on the prairies and cut the trees from the forests. By chopping large blocks of habitat into fragments, we created vast amounts of new edge and produced a population explosion among cowbirds. Their population growth over the past 40 years has been so large that they have become too numerous even for the greatly expanded habitat available to them. Animals facing such a situation often respond by invading marginal habitat, and cowbirds have done just that, moving into the forest interior where the indigenous birds have no defense against them.

Robinson’s study area includes both the central portion of the state, where the only forests are woodlots, most of them 100 acres or less, and the Shawnee National Forest at the state’s southern tip, where some blocks of forest cover nearly 1,000 acres. “From reading the literature,” Robinson told me, “we had expected that the larger the forest, the smaller the problem from cowbirds. But some species are doing badly pretty much wherever they are.”

The birds facing the greatest danger are all long-distance migrants that winter in Central and South America. They are especially vulnerable because they spend only a short time here, in most instances nesting only once each summer. Resident species, such as cardinals, and short-distance migrants, such as robins, nest several times a summer. Their chances are improved by the fact that cowbirds seem to leave the forests by July, so late-summer nests are not bothered. A few long-distance migrants, notably the indigo bunting and the white-eyed vireo, also nest several times a summer, and these species are doing all right.

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.

Other species showing signs of serious parasitism include summer tanagers, scarlet tanagers, and yellow-throated warblers. Cerulean warblers may also be affected. “We have at least 15 study areas in southern Illinois that contain appropriate habitat for them,” Robinson says, “but we have ceruleans in only three of those areas and only in very low numbers.”

Cerulean warblers nest in treetops, making it very difficult to study their nests. In addition, they are facing heavy pressure on their wintering grounds–virgin forests in the Andes at just the altitude that is ideal for growing both coffee and coca. They are losing winter habitat at a high rate.

Robinson emphasizes that a big forest in Illinois is still not very large. “In really big tracts in Wisconsin, in the central Ozarks, the Smoky Mountains, and the White Mountains in New Hampshire we know that cowbirds are not a problem. It may be that 1,000 acres is not big enough for southern Illinois.”

Robinson suspects–although after finishing only one year of the study he cannot say anything for certain–that the age and structure of the forest also contribute to the problem. Much of the Shawnee consists of uniform plantations or of old farms abandoned during the depression and now grown up to even-aged forests. “I think the cowbirds have learned how to find nests in these simple, homogeneous forests, where there are only a couple different kinds of places where birds can hide their nests,” he told me. “We may need a large block of forest with a variety of habitats within it.”

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.

The implications of his studies–and the many others that have shown that fragmented habitat does not support the rich diversity of our native bird life–should be pushing us toward changes in the way we use the land. I must admit that for someone with my political and social biases, it is pleasing to learn that Chicago is not only a more exciting place to live than, say, Lake Forest, but also ecologically better. Scattering millionaires around the landscape may be visually pleasing, but it uses up a tremendous amount of land, converting a big woods–or prairie–into a collection of fragments. Incidentally, lawns are ideal feeding grounds for cowbirds, and shade trees make excellent perches.

Two land-use battles now going on in the Chicago area plainly demonstrate the difference between the way we have thought and the way we need to think. In Highland Park, a developer wants to put some expensive houses on a high-quality prairie remnant. A group of local people is opposing him. The developer’s defense of the project emphasizes that much of the land would be preserved, that the houses would be widely scattered in an aesthetically pleasing way. However, this style of building has a heavier impact than building a single high rise in one corner of the property and leaving the rest of the land alone.

For similar reasons the idea of putting a living-history farm on Cook County Forest Preserve land in Orland Park ought to be canceled altogether, or at least rethought. While no definite plan has been drawn, backers of the project are saying that the separate units of the farm would be scattered over the land to reduce their impact. But scattering would have the opposite effect. Reducing the impact would require putting everything in the smallest possible space in one corner of the property.

If nature is to survive, we need to give it some space.