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The first ever Cook County Owl Census will be taking place this weekend. We already have a breeding-bird survey that takes place in June, but that survey tends to miss owls both because the surveyors usually work in the daytime and because owls nest very early in the year; by June the peak of the season is past, and the animals are much less conspicuous.

Chicago is within the breeding range of no less than seven species of owls, but the census takers are likely to encounter no more than two: the great horned and the eastern screech. These are the species that have managed to cope with the radical changes we have made in the landscape. Just how well they are coping is one of the major questions the census-- which will become annual--will try to answer.

Screech owls are small birds. In overall length, they are no bigger than robins, although they are somewhat more robust in build. They like open areas with scattered trees more than dense woods, and if they aren't persecuted, they will happily nest in the big backyards of affluent suburbs.

Although they are small, they are fierce predators whose diet can include large insects; mice and other small rodents; small birds, snakes, and lizards; and even other screech owls.

Great horned owls tend to prefer woodlands, but they have the sort of flexibility you might expect from a bird whose breeding range extends from the Arctic Ocean to the Strait of Magellan. The claim has been made, and nobody has refuted it yet, that the great horned owl is the only bird that nests in every county in the continental U.S., including Alaska.

Pound for pound, great horned owls are the champion predators of North America. Their diet includes rats, mice, rabbits, skunks, house cats, rattlesnakes, birds up to goose size, and, of course, screech owls. Screech owls tend to be very circumspect when they know there is a great horned owl in the neighborhood.

There are several techniques for finding owls. In daylight, you can listen for crows. If a crow sees an owl--or a hawk--it starts cawing a special mobbing call that attracts other crows. The flock gathers around the owl, perching close--but not too close--and hollering. If the owl moves, the crows will follow.

Mobbing seems to neutralize the owl. You can't sneak up on anything when 15 screaming crows are following you around.

A flock of four crows chased a great horned into my backyard about a month ago, providing me with a new species for my backyard list, one I never expected to get. I live near the north branch of the Chicago River, and my owl was undoubtedly one of the birds that moves up and down the river to hunt. It flew off in the direction of the river trailing a crowd of noisy crows.

After dark the best way to find an owl is to imitate its call. You can do this with a tape recorder, using prerecorded calls, or you can learn to make the noise yourself. Using a tape always seems like cheating to me. It's like lip-synching to a record or being an Elvis imitator.

Imitating an owl is not really all that difficult. If you can manage a high-pitched whistle while vibrating your tongue like a trumpet player doing a show-off solo of "Carnival in Venice," you can do a passable screech owl. A great horned owl sounds like somebody blowing across the mouth of a gallon jug that's about a third full.

The screech-owl call may do the trick. Great horned owls often come to investigate screech-owl calls. They don't like screech owls in their territory, and if they find one they kill it.

In the daytime, screech-owl calls will also attract small birds like chickadees that come to mob a predator that really scares them.

Calling is most effective during the nesting season, when the birds are establishing and defending a territory. A calling owl is declaring his ownership of a piece of ground, and he will be quite aggressive about driving off potential competitors.

Sometimes you can find screech owls by banging on the trunks of hollow trees. Screech owls nest in holes in trees, and if you knock on the trunk, a bird may stick its head out of its nesting hole to see who's there.

The five species of owls that could nest in Cook County but probably don't are the barred owl, the barn owl, the saw-whet owl, the long-eared owl, and the short-eared owl. All of these are absent because we no longer have the proper habitat for them.

Barred owls prefer to live in swamp forests or in dense woods on river floodplains. They nest along the Des Plaines River in Lake County, and on rare occasions a Lake County bird might slip south across the county line, but they haven't been known as nesting birds in Cook County for decades.

There is no obvious reason why the Cook County Forest Preserves along the Des Plaines have no barred owls while the Lake County preserves a few miles north on the same river have a well-established population. The likeliest reason is just that the Cook County preserves are hemmed in too closely by development.

The barn owl is an endangered species in Illinois. Again, habitat loss plays a major role. It may seem strange that Illinois would have a shortage of barns, but the old-style wooden barn with a hole or two under the eaves big enough to admit a 20-inch bird is passing away. The galvanized-metal sheds that are replacing them lack accommodations for owls.

The Illinois Department of Conservation is putting out owl houses here and there, but the effort has a certain forlorn quality about it. It's like somebody putting a light in the window for a sailor who vanished in a storm five years ago and has never been seen since. There are so few barn owls left in Illinois that there is almost no chance of one finding a house.

The saw-whet owl was probably always a rare bird in northern Illinois. It is a bird of northern forests, and Chicago lies at the extreme southern edge of its nesting range. We do have evidence of two nests in 1982, but none since. We see the bird regularly in winter.

The name comes from its call, an endlessly repeated whistled note that reminded somebody of the sound of a saw being sharpened. Since I have never heard a saw being sharpened, I can't say if the name is appropriate.

Saw-whets are tiny birds, only seven or eight inches long, and they often seem almost suicidally tame. They will sit on a low branch and allow a human to get practically nose to nose--or nose to beak--with them.

Long-eared owls look rather like slightly scaled-down versions of great horned owls. (I assume everybody knows that the "ears" and "horns" of owls are actually just tufts of feathers. An owl's ears are on the sides of its head, and they do not stick out.) Long-eared owls are birds of dense woodlands with a strong affinity for conifer groves. The most recent nesting record for Cook County was in Eggers Woods on the southeast side in the early 60s. There are more recent records for northern Indiana.

Short-eared owls are open-country birds that often come out to hunt during the day. They hunt by flying low over the ground with a light, buoyant, almost mothlike wing beat. They will hunt over uplands, but they prefer low ground, sedge meadows, wet prairies, and marshes. I am told that about 30 pairs of short-eared owls breed every year in northeastern Illinois, but none of them nest in Cook County.

People involved in conserving natural lands are often concerned with the importance of preserving the diversity of nature. Our owl situation demonstrates what they are talking about. A century ago, we might have found seven species of owls nesting in Cook County. Today we can expect to find only two. The rest are lost mainly because they have no suitable place to live. The loss happened despite a remarkable conservation effort that has placed almost 11 percent of the county in forest preserves. We have drastically reduced the natural variety of life in this place, squeezed out much of the diversity and created a dulling sameness in its place.

Owls are predators, animals at the top of a food chain whose lower links are made up of smaller animals and hosts of plants. If short-eared owls have disappeared, we must fear that many of the plants and animals that once sustained them are gone too.

If you have time this weekend to search for owls, you can report your observations by mail to the Chicago Audubon Society, 5801 N. Pulaski, Chicago 60646, or by phone at 539-6793.

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