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At least 110 species of birds nested in Cook County in 1990. Five of them were exotics: pigeons, starlings, house sparrows, mute swans, and monk parakeets. The other 105 were natives.

We have these numbers thanks to an army of 196 volunteer observers who devoted free time in spring and early summer to searching parks, forest preserves, and other likely areas for signs of nesting. And we owe the size of that army to Alan Anderson of the Chicago Audubon Society. Anderson is determined, indefatigable, and a relentless nagger. He lines people up and sees to it that they do the job--including the dull but necessary paperwork that allows him to tabulate the results.

The immediate purpose of this survey is to gather information for the "Illinois Breeding Bird Atlas," a joint project of the Illinois Department of Conservation and various birding and environmental organizations around the state. Organized by county, the Atlas will give us a comprehensive look at the nesting ranges of Illinois birds.

Each county is divided into a series of blocks measuring two and a half minutes of latitude by two and a half minutes of longitude. That translates to a block about three miles east to west and two and a half miles north to south. There are 120 such blocks in Cook County, 105 of which were covered this year. The ones left unsurveyed were all unpromising city areas that probably wouldn't have given us any additional species.

Observers are supposed to place what they see in any of four categories. "Observed" is the most trivial. It just means that you saw a bird of a particular species during the nesting season. The hierarchy rises through "possible nester," to "probable nester" to "confirmed nester."

To confirm nesting you need to see a nest occupied by young, eggs, or an incubating adult; recently fledged young still incapable of sustained flight; adults feeding young or carrying food to a nest; or nest building by adults of any species except wrens and woodpeckers. Wrens and woodpeckers building nests don't count because male wrens build dummy nests that are never used, and woodpeckers may excavate several holes in trees before finding one they like.

The atlas is not a population study. The idea is to confirm nesting for as many species as possible. So once you are sure that you have at least one pair of nesting robins, for example, you move on to other species.

The completed list of Cook County's nesting birds is full of interesting stuff. Consider the endangered and threatened species. The pied-billed grebe, for example, nested in 14 census blocks. Most of these were the kinds of places you might expect to find this water bird: marshes at Lake Calumet and in the forest preserves. But pairs also successfully produced young in Washington Park and in Sherman Park, at Garfield and Racine.

The endangered double-crested cormorant nested at Baker's Lake in Barrington and probably somewhere in the Palos forest preserves as well. Black-crowned night herons were in their usual places at Lake Calumet and Baker's Lake, but they also probably nested along the North Shore Channel north of Touhy Avenue.

Mute swans raised young in at least three locations in the county, and the uncommon hooded merganser bred in Glenview and at Crabtree Nature Center in Barrington .

We had three pairs of Cooper's hawks, at least one nesting pair of red-shouldered hawks, a pair of broad-winged hawks, and at least 16 pairs of red-tailed hawks.

The threatened common moorhen successfully nested in three different parts of the county: near Lake Calumet, at McGinnis Slough in Orland Park, and in Schaumburg.

The big colony of ring-billed gulls and herring gulls at Lake Calumet appeared to be even bigger this year, and a pair of herring gulls briefly incubated eggs near Crabtree Nature Center before abandoning the nest. The endangered black tern nested in Schaumburg but not in the Lake Calumet marshes, where it has been at least an occasional nester for many years.

All six of the swallows found in eastern North America nest in Cook County, and Carolina wrens seem to be making a comeback. Carolina wrens are sedentary birds. They stay in the same general area year-round rather than migrating south in winter. We are at the very northern edge of their range here, and the cold winters of the late 70s wiped out the local population. Our recent mild winters have allowed them to recolonize this area. There were five confirmed nests; four at the southern end of the county and one in Bemis Woods, which is along Salt Creek at the western end of La Grange Park. Several individual birds were seen in various other places around the county.

Eastern bluebirds are another species making a comeback, thanks in considerable part to the creation of bluebird trails: a line of nest boxes placed at suitable intervals along a path. Bluebirds had declined, in part at least, because they lost both habitat and nest sites. They nest in cavities, and the arrival of the starling presented them with a formidable competitor. The nest boxes are made with entrance holes that are just slightly too small for starlings but big enough for bluebirds. We now have nesting pairs in 13 of the census blocks, and last year they produced at least 60 young.

Five pairs of nesting wood thrushes were reported, but I don't know how successful they were. Wood thrushes are birds of forest interiors, and elsewhere in Illinois they have been heavily parasitized by cowbirds. The cowbirds lay their eggs in the thrushes' nests, and the thrushes raise the cowbird young, usually at the expense of their own offspring. Cowbirds used to be confined to woodland edges and grasslands, so forest-interior species like the wood thrush have developed no defenses against them. With their habitat increasingly fragmented, wood thrushes are facing an entirely novel threat.

The status of all long-distance migrants is a matter of great concern in North America. Long-distance migrants spend their summers in the temperate zone and their winters in the tropics. In addition to thrushes, the category includes vireos, wood warblers, and tanagers. They are catching hell at both ends of their migration routes. Habitat loss both here and in the tropics and heavy predation and cowbird parasitism are all conspiring to reduce populations.

Our vireo populations don't look too bad. Warbling vireos were confirmed nesters in 13 census tracts, and red-eyed vireos nested in 15. Bell's vireo and the white-eyed vireo were both scarce, but these species are at the edge of their range here so that's to be expected. The yellow-throated vireo, however, which is not at the edge of its range, was confirmed in only two blocks.

Among the warblers, things looked even bleaker. Only the yellow warbler (26 blocks) and the common yellowthroat (21 blocks) were present in any numbers. We had two pairs of blue-winged warblers and one each of black-and-white warbler, cerulean warbler, American redstart, and yellow-breasted chat. And that was it. There were some singing males of other species, but no proof of nesting.

Our most common sparrow, to nobody's surprise, was the song sparrow, followed by chipping sparrow and field sparrow. We had two confirmed nesting pairs of grasshopper sparrows, but no confirmed nesters among the Henslow's sparrows. However, there were singing males of this species in three locations. Both grasshopper and Henslow's sparrows are prairie birds whose status is always a matter of anxiety, since nearly all their natural habitat is gone.

Orchard orioles nested in four blocks, although one adult was observed feeding a young cowbird, so its nesting efforts may have been in vain. And the house finch, never seen here before the past decade, is now represented by 149 nesting pairs in 45 census blocks.

Anderson and his army of volunteers deserve great credit for gathering all this information. We now have the most complete picture ever assembled of the bird life of this county. Of course, this year's census is just the beginning. What we really need is continuous monitoring of the status of our breeding birds. Only that will allow us to make sensible judgments about which species are doing well and which ones are in trouble. Birds are also excellent indicators of the general state of our environment. They are mobile enough to respond quickly to changes and visible enough to be easy to count.

The census also helps create a new kind of relationship between us and our native land. Like the volunteers who are rebuilding our prairies, the census takers are forging a link with nature that is based on love rather than exploitation. As we approach the 500th anniversary of Columbus's first voyage, these birders are a sign that we are at last settling down here and making this land our home.

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