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A pair of bluebirds is nesting at North Park Village. Laurel Ross, the naturalist in charge of the nature preserve at Pulaski and Peterson on the site of the former municipal TB sanatorium, called me Monday morning with the news.

"We have had the bluebird boxes out for three years," she said, "but all we ever got were house wrens and chickadees. I really never expected this."

The nature preserves at North Park include a 46-acre plot at the north end of the property and 18 acres at the southeast corner, but the bluebird houses are scattered throughout the property, which measures a half mile on a side. This pair has moved into a house on a mowed lawn that is part of the green space left between buildings. The volunteer who monitors the houses discovered four eggs in the box when she checked it Monday morning.

Bluebirds nest exclusively in cavities, rather than in the open nests used by their close relatives the robins. They used to be common birds until their traditional way of life fell victim to a slew of assaults. House sparrows and starlings--both alien species introduced by Europeans into North America--began to compete with them for nest sites. Farmers scrapped their wooden fence posts in favor of steel poles, so the natural cavities that decay opened in the wooden posts were no longer available. Then the grasslands along the gulf coast that were the species's main winter home were converted to cropland. Bluebird populations collapsed.

Lately they have been making a modest comeback, thanks in part to the bluebird trails--groups of nest boxes scattered through suitable habitat--that bird lovers have erected to give the species a hand. In Cook County bluebirds nest regularly on the bluebird trail at Crabtree Nature Center in Barrington, and in boxes provided for them in the Ryerson Preserve in Lake County, among other places.

At Somme Woods Preserve in Northbrook, where I've been doing a survey of the nesting birds, there are no artificial nesting boxes but the supply of natural nesting holes is large and growing. The North Branch Prairie Project is restoring the natural landscape at Somme, a landscape that mixes prairie with savanna dominated by oak and hickory trees. Invasive trees, species like box elder, cottonwood, and green ash, are gradually being girdled. Girdling involves cutting away the outer layer of the trunk in a narrow band that runs all the way around the tree. The cut severs the connection between roots and leaves and kills the tree.

The trunk, however, remains standing, providing food for wood-eating beetles and other such creatures that in turn provide food for woodpeckers and other insectivores. The woodpeckers dig nesting and roosting holes in the dead wood, providing homes for chickadees, great crested flycatchers, and eastern bluebirds.

The pair of bluebirds I first saw at Somme on May 29 were inspecting a nesting hole dug by a woodpecker in the stub of a tree trunk. Apparently, the hole did not measure up to their specifications; they didn't nest there. I think it will take some blind luck to discover just where they did nest. It is easy to find bluebird nests in boxes built for the purpose, but very difficult to find nests located somewhere on a 150-acre preserve loaded with standing dead timber. The birds are still around someplace, but they are elusive.

I did find a red-tailed hawk nest, though--a discovery that has been one of the highlights of my survey at Somme even though I have come to expect sighting a red-tailed hawk almost every time I visit the western part of the preserve. Red-tailed hawks are not exactly rare; in fact they are the only large hawks doing well enough to be considered common in North America. They nest from Florida to Alaska and from Hudson Bay to Central America. Any animal with a range that large is likely to be an adaptable species, and red-tails are certainly that.

In eastern North America, they do quite well in a landscape that mixes open land--where they like to hunt--with patches of woods where they can put their nests. The western half of the Somme Preserve is exactly the sort of place they favor, since it has both prairie and dense second-growth woodland.

Sometimes I see them soaring. Red-tails have wingspans of four feet or more, and their broad wings and tails are beautifully adapted for riding winds or sailing on updrafts. They can hang in the sky for hours with almost no effort, letting the air supply the energy they need to stay aloft.

Often I see them perched in one of the few tall trees that border the largest patch of open prairie at the preserve. They could be surveying the ground around them, looking for something to eat, or they could have full bellies and just be resting.

Most people know about the acute vision hawks possess. The secret of their sharp eyes lies in the large numbers of receptor cells in their retinas. The dense concentration of these cells--there are several times as many per square millimeter as there are in human eyes--provides them with a much sharper image of the world than we get. The difference is like the difference between the grainy reproduction of photographs in newspapers and the images in National Geographic or the Playboy centerfold. The sharper image enables a soaring hawk to spot a mouse scurrying through the weeds, a movement we could see only from very, very close.

Red-tails catch some birds, but they mainly live on small mammals. They catch and eat creatures as small as mice, but rabbits and ground squirrels are the sorts of things they prefer. It was a red-tail flying by with some kind of animal in its talons that alerted me to the presence of the nest at Somme. The bird flew across the open prairie, where I was standing, and then continued on over the woods, where I lost sight of it. A few minutes later it flew out of the woods with nothing in its claws. Obviously it had delivered the food to young hawks in a nest. So I started to search the woods.

Red-tails build their nests in the tops of the tallest trees. This time of year the dense foliage could make even the largest nest invisible from the ground, but you can find certain clues by looking at the ground rather than toward the treetops. As soon as baby hawks gain control of their movements they learn to back up to the edge of the nest and shit over the side. This keeps the nest clean--which probably cuts down on disease--and it also coats the ground under the nest with the chalky white excrement of the young hawks. It was the whitewash--as ornithologists delicately call this stuff--that revealed the nest to me.

The nest itself was huge, like a bale of hay stuck up in the top of the tree. Once I had found a place where the leaves didn't block my view, I watched it for a few minutes. At first I saw no movement. But then a nestling appeared. Standing at the edge of the nest looking down at me, it was still except for the occasional practice flap of its huge wings. Its flight feathers had already emerged, but its head was still covered with down. Based on what the books say about the development of young red-tails, I would guess that it was two weeks out of the egg, which means it would have about two more weeks in the nest before it got old enough to leave. After a few minutes I saw some movement on the other side of the nest. It was a second nestling, giving some practice flaps to its developing wings.

I left after seeing the second baby. Red-tails are notoriously shy around their nests, and it is quite likely that the parents might not bring any food to the nestlings as long as I was standing there.

I hesitated about revealing the location of this nest. I wouldn't want to be responsible for directing hordes of hawk watchers to it, who might disturb the birds so badly that they would abandon the nest and the young. But red-tail nests aren't that unusual. Besides, by the time anybody reads this, the young birds will probably be old enough to leave the nest for good.

I should add that the most abundant animal in the forest at Somme Woods is the mosquito. Anybody who wants to go on a nest hunt will find himself surrounded by clouds of buzzing insects every step of the way.

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