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Some random signs of spring: The most reliable harbinger of spring in Chicago is the changing of the signs around the North Pond in Lincoln Park. In the winter, the signs say "Danger, Thin Ice," but in spring, the Park District takes down those signs and replaces them with signs that say "Danger , No Swimming." Given our typical April weather, it seems appropriate that our sign of spring should be a warning sign.

This continues to be a good spring for bald eagles. Three immatures and two white-headed adults were seen at Saganashkee Slough in the Palos Hills two weeks ago, and there were several sightings earlier in the spring from both Palos and Illinois Beach State Park north of Waukegan.

Spring is also the time to begin my annual breeding bird survey at Somme Woods. I made my first visit last Saturday morning. I drove away from my house through a lake-effect snow storm that barely reached as far west as the Edens. Somme was enjoying cold, sunny weather with no sign of a spring storm. Returning to Somme after several months absence can be disorienting. The North Branch Prairie Project has been working on this site for close to 15 years now, and major changes have been accomplished in that time. Also, nowdays, the NBPP is so big and so successful that hordes of volunteers show up for every work day. As a result, the pace of change at Somme has really picked up. A large area of brushy, second growth woods right in the middle of the east side of the preserve, a feature so prominent I always thought of it as the Wooded Island, has been mostly cleared. The brush has been piled up for eventual burning; the bigger trees have been girdled and left standing.

Girdling trees involves removing a strip of bark from around the trunk. If you do it just right, you will cut away the phloem cells that carry food from the leaves down to the roots while leaving the xylem cells that carry water and nutrients upward. That way the roots will keep pumping stuff up to the buds and leaves, but the leaves will never send any nutrition down to the roots. A girdled tree may put out leaves for a year or two, but its roots are slowly starving. When it eventually dies, the roots will be incapable of sending up new sprouts. If you cut down a healthy tree, the roots will send up several new stems which will have to be cut in their turn.

Another advantage of girdling is that the dead trunk is still standing, providing a home and a rich food source for woodpeckers and various other birds and mammals. The number of standing dead trees--they are called snags--at Somme is enormous. We have two species of nesting woodpeckers at Somme, the downy and the northern flicker. I am hoping that some red-headed woodpeckers will take up residence too.

Woodpeckers are an ecologically important family because their powerful beaks and neck muscles enable them to dig nesting holes in snags. When the woodpeckers are through with the holes, a rich assortment of other birds and animals--chickadees, nuthatches, deer mice--take them over. None of these inheritors is capable of excavating a cavity in wood, so they are all dependent on the work of woodpeckers.

(Another sign of spring. As I was writing the previous paragraph, a flicker landed in the big ash tree just outside my office window. These birds are on the move now, and you can see them in all the parks and on any quiet street with a few trees. They are just slightly larger than robins. When they fly away from you, you can see the large white patch on the rump that makes them very easy to identify.)

The hole-nesting bird we are most anxious to find at Somme is the eastern bluebird. I saw one male bird on my visit last Saturday. Of course, I saw two males last year in late March, but I never could find any during the actual nesting season. I think they just hang around a while in early spring to tantalize me.

Bluebirds used to be very common, but a combination of habitat loss and competition from starlings and house sparrows put them in serious trouble. In recent years, people have been building bluebird trails in an attempt to get them back. A bluebird trail is a string of nest boxes widely spaced along a path through suitable habitat. The entrance holes on the boxes are too small for starlings. If house sparrows move in, the keeper of the trail just lifts the roof off and removes them.

For a considerable period, we had no nesting bluebirds in Cook County, but thanks in part to the bluebird trails, we had 27 pairs breed here last year, and there are more in Lake County just northwest of Somme Woods. So nesting eastern bluebirds are a real possibility, and also a fond hope. Bluebirds were certainly one of the typical birds of the oak savannas that were once widespread in Illinois. At Somme, we are trying to restore that almost lost savanna ecosystem, so bluebirds would add authenticity to the project.

I flushed two American woodcocks on my walk around the preserve. I will be visiting Somme at twilight over the next week or so to get a census of this bird. Male woodcocks attract mates by performing an elaborate display just as the sun is going down. They sing a long, twittering song while flying straight up, well above the tree tops. At the apogee of their flight, they cock their wings in a sort of half-open position and then float down to the ground as lightly as falling leaves. They are a real sign of spring.

Robins are everywhere at Somme right now. Several pairs will nest there, but right now, we also have flocks of migrants passing through. They love to feed on the open ground, especially the parts of the prairie that have just been burned. Apparently, they get a clear look at the ground and at earth worms in the burned patches.

Song sparrows are back--although none of them were singing. And of course, the usual red-wings and grackles and large numbers of crows were around. I saw two eastern phoebes. These will leave and not nest here. There were three blue-winged teal on Oak Pond and a pair of wood ducks in the ditch that channelization has made of the Chicago River. And a small flock of cowbirds was perched on the bare branches at the top of a tall snag. Cowbirds are brood parasites who lay their eggs in the nests of other birds. They are waiting for the song sparrows and robins to start nesting.

Deer were everywhere. I saw six on the east side of the preserve, and on the west side, a herd of seven animals were all feeding together while an eighth was browsing on its own a couple hundred yards away. We have been hearing a lot about the problems created by overly large deer populations along the Des Plaines River. I suspect the problem is almost as serious along the North Branch of the Chicago. Ultimately, the Forest Preserve Districts in Cook and Lake County are going to have to work out a coordinated management scheme for their deer populations. Otherwise, our preserves will become more and more degraded.

I saw no signs of owls on my first spring visit to Somme. I had tried to find some on a twilight visit in January with an equal lack of success. Owls start nesting in the dead of winter, so January and February are good times to search for them.

Actually, the best way to see owls may be to stop looking for them. I certainly wasn't looking for owls Monday morning when I was walking my dog through the alley between Mozart Street and California Avenue. But suddenly, I heard all these crows screaming their distinctive mobbing call. And I looked up and saw agitated pigeons flying around in the very fast circles.

Mobbing is a strategy that crows--and many other songbirds as well--use on hawks and owls. If a chickadee spots a screech owl or a crow sees a great horned owl, it starts this very excited calling. The call attracts other birds, until you have a whole flock screaming at the owl. This seems to work to the advantage of the small birds. Every predator depends to some extent on stealth, and it is hard to be stealthy when fourteen crows are all flying around you screaming at the top of their lungs.

The center of the action seemed to be on Mozart, so I hurried back to the street. The crows were buzzing something perched at the top of a tall silver maple, and when I got right under the tree, I could see something that looked like a beer keg with feathers on one of the highest branches. It was a great horned owl. For a while it sat there, alternatly looking down at me and up at the crows. The crows were still buzzing it, but they weren't getting too close. Crows are well aware that discretion is the better part of valor. Finally, the owl took off, heading southeast, probably back to the river from whence it had come.

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