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The most interesting bird I've seen this spring at Somme Woods was a transient, a male prairie warbler who sang all morning on May 26, and couldn't be found on May 27.

I'm doing a survey of the breeding birds at Somme Woods, a Cook County forest preserve in Northbrook, and a breeding prairie warbler would have been big news. This species, despite its name, is not really a prairie bird. Its nesting range excludes most of Illinois and all of Iowa. In presettlement times, it probably would have colonized burned ground that was just beginning to return to forest. In contemporary North America, it is fond of Christmas-tree farms, with their nicely spaced rows of small trees.

But in Illinois, it has rarely nested north of Peoria. In the Chicago area, its only known nesting site is at the Indiana Dunes, so just seeing the bird in the northern suburbs was a big deal. I first heard it singing at the edge of the gorgeous old oak savanna called Vestal Grove. It took me at least 15 minutes to actually find the bird, but the sight was worth the wait. I had never seen a prairie warbler in Illinois before.

I also found a Brewster's warbler singing at Somme. Brewster's warbler, named after the noted 19th-century American ornithologist William Brewster, is not a species. It is a hybrid, the result of a cross-species mating between a blue-winged warbler (Vermivora pinus) and a golden-winged warbler (Vermivora chrysoptera). The blue-winged is a southern species, the golden-winged northern; here in northern Illinois their breeding ranges overlap and they hybridize freely.

The dominant type resulting from the cross is the Brewster's warbler, which looks very much like the blue-winged except that, instead of solid yellow underparts, it has a white throat and belly separated by a patch of yellow on the breast.

The recessive cross is Lawrence's warbler, named in honor of an amateur ornithologist from New York with the euphonious appellation Newbold Trotter Lawrence. This bird looks like the golden-winged except that its underparts are yellow instead of white and its wings have a patch of white rather than gold. The Brewster's warbler at Somme was singing the song of the blue-winged warbler.

The problem all these singing males have is finding a female. Somme could provide a suitable habitat for nesting prairie warblers, but the odds are heavily against a female showing up to join a male who has somehow wandered 100 miles away from the nearest nesting ground of his species.

Sometimes the problem of males and females finding each other can reach lethal proportions. Ornithologist John Terborgh, in his book Where Have All the Birds Gone?, suggests that the Bachman's warbler may slide into extinction because a tiny population spread over a huge area leaves few opportunities for males and females to connect. The Bachman's warbler nests in swamp forests and winters, according to Terborgh, in Cuba. The conversion of Cuba's tropical forests to sugar-cane plantations, a process that began at the turn of the century, eliminated almost all the winter habitat and seriously reduced the population. When the birds return north in spring to swamp forests from South Carolina to Arkansas, the few individuals left are so widely scattered that males and females may not be able to find each other.

I suspect that the eastern meadowlark now residing in the semi-prairie at the west end of the Somme Woods preserve may be encountering similar difficulties. I first saw the meadowlark, or at least a meadowlark, on April 5. This was big news, because the meadowlark is a prairie bird, and no prairie birds had been nesting at Somme despite 12 years of prairie restoration.

On May 29, I heard the meadowlark singing from a black locust tree just south of the preserve itself. Then it flew over me and landed in a small tree on the prairie, way over at the northern end of the preserve. It stayed in that tree singing for several minutes. It acts like a bird still in the process of establishing a territory. If there were a female sitting on a nest, the male would probably not have been moving so far. However, I shall keep hoping.

I began to realize early this spring that I was trying to cover too much ground in my visits to Somme. So I have been making a serious effort to slow things down. I have divided the territory I am covering into four separate sections, and I cover only one section on each visit.

It takes patience to get a good look at wild animals. You have to "move slow and set frequent," as Annie Dillard described it in Pilgrim at Tinker Creek. I have always thought of patience as the highest of virtues, but it's not one that comes easily to me. My natural tendency is to move along, to get on with it. There could be a loggerhead shrike 100 yards down the path; why should I stop here and wait for a look at the yellow warbler I hear singing in that cottonwood tree?

But patience is an acceptable substitute for skill, so I need it in an enterprise like this. And it does bring rewards. I am amazed at how quickly wild animals will accept you as a part of the landscape if you are just willing to hold still and shut up. I am standing in an open woodland at Somme watching a pair of rufous-sided towhees. The female is hopping from branch to branch in the few small trees in the understory. Sometimes she drops to the ground. The male follows her. She lands on a branch. He lands next to her. Every time she moves, he moves too. Once in a while he sounds his call note or his three-note song, but mostly they are silent. They are never more than 20 feet from me, and sometimes they get as near as 6 feet.

I can hear a scarlet tanager singing overhead, a raspy song that sounds like a robin with a sore throat. I am tempted to look up, to raise my binoculars, to find the tanager, but I hold still. The towhees continue their stately dance. By and by the tanager flies down and lands on a low branch about 20 feet away. He perches there in plain sight, singing. Tanagers are wondrous birds with glossy black wings and orange red bodies so bright they seem to be lit from within.

And then two deer wander into my field of view. They are feeding in a relaxed way. One of them snorts from time to time. They look at me. They know I am there, but it doesn't seem to bother them. They just keep eating.

And I am getting goose bumps. It is as if I have achieved what everybody has been after, from the Teutons who dreamed of the Tarnhelm to the loonies who built the Stealth bomber: I am invisible. I am here, but nobody can see me.

So now I have to apply my patience to completing my survey. I have a list of 33 possible nesting species, but I have confirmed breeding in only two of these: the woodcock and robin. I found two woodcock nests, each with four eggs in it, and I have watched a female robin building a nest in a low shrub.

For the rest, I have only singing males. They are a start, but as in the case of my meadowlark, questions remain: Have they found a mate? Have they found a suitable nesting site?

The answer is almost certainly yes for some of my 31 undocumented species. Song sparrows, catbirds, cardinals, yellow warblers, house wrens, goldfinches, flickers, and mourning doves are so numerous it would be hard to believe that males haven't been able to find a female. Starlings have already produced their first brood of the year.

Others I can't be sure of. I have only one singing eastern wood-pewee. He may not find a mate. Least flycatchers are around and singing, but they are very late migrants and might move on. Veeries seem like a long shot, but I keep seeing this species in the same place every time I go there, so it could happen. Veeries are ground-nesting thrushes on the threatened list in Illinois. They nest regularly at Ryerson Conservation Area, just a few miles away, so maybe I'll get lucky.

I watched two flickers in seeming courtship around the trunk of a dead tree one morning. They flew from branch to branch in the live trees surrounding the snag, and from time to time landed on the snag itself, where one of them would duck into a hole dug in the trunk. Well, I thought, they are picking out a nesting site. And then I noticed that both of them were males. Good God, what if Jesse Helms finds out about this.

My wife says the PCBs are the problem. PCBs do suppress the production of sexual hormones, and birds with heavy PCB concentrations often display ambiguous sexual behavior. They seem unable to decide whether they are male or female, and if they successfully produce young, they may do an inadequate job of caring for their offspring. I shall try to keep an eye on these uncertain flickers.

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