Suddenly we are getting all sorts of woodland birds at Somme Woods, the forest preserve in Northbrook where I am conducting a survey of nesting birds. We have a singing male wood thrush that has been hanging out in the same territory for at least five days.

Right next door to him is a singing male veery. We may have as many as six singing red-eyed vireos. A week ago Tuesday I discovered the nest of a pair of blue-gray gnatcatchers, and last Sunday I found a singing male black-and-white warbler.

These are all unlikely birds for the habitat we have. Fifteen years ago Somme was a mixture of old fields containing scattered prairie remnants, oak savannas infested with the invasive shrub European buckthorn, and truly junky second-growth woods.

In the last 15 years intensive efforts by the North Branch Prairie Project have improved and expanded the prairies, removed most of the buckthorn from the oak savannas, and decreased slightly the area covered by the second-growth woods.

But junky as they are, the remaining second-growth woods are apparently attractive enough to at least a few forest birds. When I say these woods are junky, I don’t mean there’s a lot of trash lying about–though in places there is. I mean that what we have here is a low-rent plant community that would cause any self-respecting botanist to throw up his hands and then his lunch.

The trees are mostly the usual invasive types that colonize open ground in these parts: green ash, box elder, here and there a few tall cottonwoods that must have been the very first trees of this forest. Quaking aspen grows in parts of this forest, along with a few trees of an alien species of poplar the Forest Preserve District claims to know nothing about–though there is evidence that years ago it deliberately planted some.

Under the canopy grows a dense shrub layer of buckthorn and other nasty aliens such as Amur honeysuckle and multiflora rose. And the ground is infested with garlic mustard, another alien that is establishing a nasty reputation in woodlands all over northern Illinois. Almost the only visible flowers, apart from the honeysuckles, are some sort of domestic phlox that has become established in patches along the west fork of the North Branch of the Chicago River, the channelized stream that splits the preserve in half.

So what is a high-class bird like a veery–a threatened species in Illinois–doing in a place like this? One answer is that it doesn’t have much choice. High-quality woods are scarce in northeastern Illinois. Another answer is that the veery doesn’t care about the botanical quality of these woods. It has other ways of judging whether or not this will be a good place to live.

A few years ago Chandler Robbins, a research biologist with the U.S. Fish and Wildlife Service, directed a study of the nesting requirements of forest birds in the eastern U.S. Working in several adjacent counties in Maryland, Virginia, and Pennsylvania, Robbins and his colleagues censused woodland birds in tracts ranging from tiny woodlots of ten acres or less up to huge tracts of forest covering several thousand acres.

While they were counting the birds, they also surveyed the forests, examining 15 variables they thought might affect the distribution of the birds. They looked at the overall size of the tract and at how much of the area within two kilometers of the bird’s territory was forested (in other words, they checked whether the bird lived in the heart of the forest or near the edge). They looked at canopy height, tree size, tree density, and the ratio of hardwoods to conifers in the forest. They plotted the position of the forest on a moisture gradient: was it a dry upland forest of oak and pine, or a wet, lowland forest of red cedar? And they surveyed the understory to measure the density of the shrubs at various heights.

When they combined their information on bird distribution with the data on the vegetation, they got a clear idea of what each species was looking for–or avoiding–in seeking a place to live. The size of the forest tract was, not surprisingly, a very important factor. The long-distance migrants, birds like the wood thrush that winter in the tropics and nest in the mid-latitudes, tended to be most sensitive to this. They concentrated in the larger tracts. Short-distance migrants, like blue jays and catbirds, were more evenly distributed; and some residents–downy woodpecker, black-capped chickadee–were as likely to be in a five-acre woodlot as in a large forest. But no species was more likely to be in a small woodlot than in a large forest.

Great crested flycatchers, which also nest at Somme, showed an unusual pattern of distribution. They reached their maximum population density in woods of about 150 acres and got less common in bigger woods. They are really more of an edge species than a forest-interior species, so the pattern is not surprising, even if it is unusual.

According to Robbins, wood thrushes reach their maximum population density in woodlands of a thousand acres or more. We are about 970 acres short of that figure at Somme. However, Robbins’s numbers show that this species can nest in woodlots as small as 2.5 acres. All the information we have suggests that nests in these very small woods are rarely successful. They are very likely to be destroyed by predators or heavily parasitized by cowbirds. It may be that the big woods produce a population surplus, forcing some birds to try to make it in little patches of trees. The birds of the woodlots will probably die without reproducing, but a new generation of immigrants from the big woods will be there to take their place. Our Somme wood thrush is almost certainly in that position.

Veeries are another species of thrush, very similar in appearance and habits to the wood thrushes. In fact, the two species are aggressive toward each other; territorial males of one species will drive away males of the other species as if they were competitors.

Both of these birds are beautiful singers. Thoreau said the song of the wood thrush was the most beautiful sound in the woods, but the eerie beauty of the veery’s song sounds much better to my ears.

Robbins discovered many similarities in the habitat requirements of the two species, but there were differences as well. One had to do with how wet the woods were. Veeries are generally regarded as partial to floodplain forests, where the soil is wetter than the ground in the upland sites the wood thrush favors. Veeries also like a dense layer of shrubs near the ground. Their preference could have to do with the fact that they build their nests on or near the ground, while wood thrushes favor nesting in shrubs around ten feet up. We certainly have a lot of low shrubs at Somme, and the veery is singing from a low area right next to the meandering old channel the river followed before the Corps of Engineers confined it to a ditch.

The singing of these two species of thrush has inspired me to spend more time in the woods at Somme. There is a lot going on in there. The singing male black-and-white warbler I discovered last Sunday is a bird I always hoped but never expected to find. In recent years Cook County has had only one known nesting site, and that is at Sand Ridge in the southern suburbs, a place where the thin, high song of this bird will soon be drowned in jet noise.

Our lone black-and-white warbler may be a long way from finding a mate and settling down, but our blue-gray gnatcatcher is already a sure thing. These tiny birds–only four and a half inches long, and most of that is tail–seem almost insanely bold. They sing, they call, they expose themselves in the tops of bare dead trees. It’s hard to see how they could last a week.

Their nest is a delicate little structure with an outer surface decorated with lichens. It sits on the limb of a hawthorn shrub, bound to the branch with spider’s silk. You can see it from 50 feet away. I can’t imagine how they will escape the cowbirds, but we will continue to hope. The only other known nesting locations for this species in Cook County are in the far-south forest preserves, at Palos and Plum Creek, and in the northwest panhandle at Poplar Creek. I hope they can get established at Somme.