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Our gray squirrels are looking plump and sleek this time of year. Their fur is luxuriant, by squirrel standards, their tails broad and bushy, and as they leap about, their bellies hit the turf with every bound.

City squirrels live by the same seasonal rhythms as wilderness squirrels, and ours are getting ready for hard times. Since late last summer, any squirrel who could get it has been packing away something like 10 to 20 percent of its body weight every day.

Squirrels have the eclectic palate typical of smart city dwellers. They eat nuts and seeds, of course, with acorns a special favorite, and they like all kinds of fruit, including tomatoes. Insects are a big diet item in the summer. Last year, the protein-rich meals created by the emergence in the east of 17-year locusts fed a bumper crop of baby squirrels.

In the city, they have been seen in parking lots picking the dead insects from front bumpers. They have learned to get inside various candy-and-potato-chip vending machines, and they are equally adept at getting out with the goodies.

They rummage through garbage cans; they steal from bird feeders; and, of course, they get fed, sometimes in staggering amounts. When I birded in Lincoln Park almost every morning, I used to see regular feeders on their daily rounds. They would usually be carrying a couple of shopping bags full of peanuts, birdseed, and stale bread, and they would scatter the food in the same places every day.

The squirrel feeders can make a huge difference. The National Park Service, which runs the city parks in Washington, D.C., had a problem with too many squirrels in Lafayette Park, which is right across the street from the White House (no jokes please). As many as 200 squirrels were running through the historic trees of the seven-and-a-half-acre square; 35 would be a more reasonable number. So the park service's Center for Urban Ecology did a study, with help from Professor Vagn Flyger of the University of Maryland, of the squirrels of Lafayette Park and how they lived.

They lived on handouts, by and large. The two most active feeders at Lafayette Park brought in as much as 100 pounds of peanuts a week between them, and thousands of less single-minded folks were sharing their lunch or their bag of popcorn with the friendly little rodents.

At first, Flyger and the park service's David Manski thought that the park's isolation was a major cause of this local population boom. Lafayette Park sits in the middle of some very broad and busy streets, so maybe the squirrels just couldn't get away.

They discovered that even two blocks of continuous pavement weren't enough to pen in a squirrel. And they also found squirrels very adept at crossing streets. "They are very alert," Flyger told me. "They watch the traffic. A few young ones get killed, but not the more experienced animals."

Some Lafayette Park squirrels crossed Pennsylvania Avenue and moved onto the White House grounds. Some moved to other nearby parks. But most of them stayed, not because they couldn't leave, but because they didn't want to. Why go wandering off when there is plenty to eat right here?

Squirrels have adapted so well to the urbanization of North America that it sometimes seems as if word of the changes down below had not yet reached the treetops. In a rich woodland, you would find about two squirrels per acre. In the city, normal populations can reach five an acre. People scattering peanuts is one of the reasons for the difference; another is the great productivity of city trees. If a city tree is not too heavily stressed by concrete and pollution, it is perfectly situated to do well. Its competitors are kept at a decent distance. Often it grows in the middle of a lawn that is dosed with fertilizer every year. City trees routinely produce large seed crops. Forest trees do so intermittently.

So the city animals are often somewhat more numerous, but otherwise they live and mate and, in all other ways, behave just like squirrels in a virgin forest, with the important exception of their tameness. Squirrels in places like Lafayette Park will eat out of your hand. Squirrels in the wild prefer to stay out of sight, and they are very good at it.

The eastern gray squirrel is the standard urban squirrel east of the Great Plains, but we have another species here as well. The fox squirrel is bigger than the gray squirrel and quite different in coloring. Gray squirrels are mostly gray. They have a patch of white on their bellies and, in winter, white tufts between their ears. Fox squirrels have tawny underparts. The sides of their faces are ocher, and their brown body fur is lightly brushed with the same color.

In presettlement Illinois, the fox squirrel was probably the more common animal. Gray squirrels like dense forests, which in Illinois would have been confined to the riverbanks. Fox squirrels like more open woods. They would have lived in the savannas of Illinois, the groves of widely spaced oaks and hickories that were scattered through the prairies on the uplands.

In Chicago, fox squirrels seem to be restricted to the bigger and wilder parks. I know of two places where you can see them. One is the wild bird sanctuary in Lincoln Park at Addison, and the other is along the river in Homer Park, between Irving Park and Montrose.

This is a good time of year to watch squirrels. With the leaves off the trees, you can follow their movements easily. This is also the beginning of the mating season, which means that squirrels will be chasing each other through the trees, showing off their acrobatic skills and giving humans a great show.

Squirrels rival monkeys in their climbing ability, and they share several traits with our fellow primates. They locomote through the trees, running along the tops of branches instead of swinging from limb to limb like apes; they use their large tails for balance and as parachutes; and when they stop to eat, they sit, bodies upright, holding the food in their forepaws as they eat it.

The prolonged courtship chases may involve several males chasing after one female. Eventually one or more of the males will mate with the female. After that, she is on her own. She has to find a den--such as a hollow tree--or build a leaf nest where she can give birth. The young, usually two or three to a litter, are totally helpless. Their eyes don't even open until they are five weeks old. After ten weeks, the female weans them and leaves the nest. By that time it's spring, and the young squirrels are on their own.

Squirrels live in dens or nests year-round, not just when they are caring for young. A good nest, like a fat belly, is an essential defense against winter. No squirrel could survive the cold and wet.

The squirrels in Lafayette Park, meanwhile, were damaging the historic trees of the square by pulling off huge numbers of leaves to make nests and by the general scratching and banging that squirrels do. To preserve the park as a historic site as well as a pleasant bit of green in the city, the park service had to reduce the number of squirrels.

The obvious solution--shoot them--was not considered feasible given the location. They did manage to reduce feeding through a program of public education. They even tried live trapping and relocation. This was a bow to the sensibilities of the times; relocated animals usually end up dead anyway. It's hard to find a woodland or a park where there aren't already all the squirrels there's room for.

Time and numbers may be solving the problem. Parasites and diseases sweep through dense populations quickly because the animals have so much contact with each other. In 1985 and again last year, winter die-offs removed a substantial portion of the population. Autopsied animals showed signs of extensive stress--another result of crowding--and the presence of multiple disease organisms. The squirrel population is now down to 35 to 40 animals, just about where it ought to be. Will it stay at that level or will it start up again? We shall see.

A birding note: This looks like a major irruption year for the snowy owl. These big raptors of the tundra come south from time to time when their northern food, mainly lemmings, gets scarce. Sightings are being reported from all over the eastern United States and southern Canada. Locally, birds have been seen in several places along the lakefront and even in parking lots at O'Hare. Watch for them.

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