This time of year, when the trees are bare, you can begin to get an idea of how many squirrels live in your neighborhood. Look for a big ball of leaves wedged into a notch between branches. The leaves are the exterior of a squirrel’s nest, and each nest is home to one adult squirrel. In a few weeks the first young of this year will be born, so some of the nests will be home to a mother and her babies.

Not all squirrels make nests of course. Some look for shelter from the cold in cavities in tree trunks, and in cities others stay warm in attics, garages, and other havens inadvertently provided by humans.

I started to think about squirrels’ nests last month when my brother-in-law pointed out how amazing it is that those balls of leaves stay anchored in the trees in the face of Chicago’s fierce winter winds. “The stems on those leaves are not twister ties,” he said. “How do they keep them together?”

I started looking at the Illinois Natural History Survey, where Edward Henske gave me an inside view of what he and other mammologists call a “drey,” a word of obscure origins.

The first point is that the leaves we see on the outside of the nest are not the structural supports that hold the thing together. Like Chicago’s other famous Sullivan, Louis, squirrels use an internal skeleton to support their structures. Louis made his of steel. Squirrels use sticks and twigs and, of course, the living branches of the trees where their nests are located.

On this framework they weave a lining incorporating grasses, strips of bark, and shredded leaves. The best way to shred a leaf if you want to weave it into a structure is to skeletonize it. This means removing most of the blade of the leaf, leaving the sturdy central vein with just a bit of blade on each side of it.

Other authorities offer somewhat different accounts of how a squirrel builds its nest. A book called The Wild Mammals of Missouri says that “the leaf nest consists of a rough twig framework, from 12 to 20 inches across, and a bulky pile of leaves heaped layer upon layer. The squirrel hollows out a nest cavity in the center of the leaves.”

However, the authors agree that the outer leaves are woven into the twig framework, and they agree that material other than leaves and twigs–they mention “grass, roots, moss, corn husks, and other items”–may be woven into the structure.

A squirrel can build a drey in about 12 hours. In other words, in a pinch a squirrel evicted from a nest in the morning could have a new shelter built by nightfall. Nests usually last six to ten months, which means that a nest built in fall will last through the winter. But nests kept in good repair have been known to endure for two or three years.

The squirrel enters the nest through a hole in the side, and the hole is likely to be on the lee side to keep the wind and rain from coming in the door. Inside there is more or less one squirrel’s worth of room. The animal curls up into a ball with its tail covering its face to maximize its insulation.

Dreys will keep a squirrel comfortable down to about 15 degrees Fahrenheit. Below that they will notice the cold, though they can survive a typical Chicago winter in a drey. Some squirrels might not make it through our coldest winters if they can’t find a good tree cavity or somebody’s attic.

Natural cavities in trees often form where large branches have broken away from the trunk. They can be enlarged by woodpeckers or by decay that enters through the hole. The tree grows new bark every year, and in the absence of any interference it will eventually seal the cavity. Squirrels keep their holes open by gnawing away the new growth.

January is breeding season for our local sciurids, to give them their official name, and if you pay attention you’ll see lots of chasing about, hollering, and a certain amount of fighting. The usual stuff associated with romance. The young will be born six to seven weeks after mating. There will be another mating season in May and June.

We have two different species of squirrels in Chicago. Gray squirrels are the typical neighborhood squirrel. They are gray, which you may have already guessed, but you may find highlights of chestnut, cinnamon, or even orange in their fur. Their bellies are white, and their tails are fringed with white.

Fox squirrels are a bit bigger, and their underparts are reddish. I know of two populations of fox squirrels in the city. One is in Lincoln Park at the bird sanctuary behind the totem pole at Addison Street. The other is in Horner Park along the river between Irving Park and Montrose. There may well be others. In fact, if you know of any others, drop a line to the Reader. Maybe we can get a population survey going.

The distribution of fox squirrels and gray squirrels in Illinois is an unsolved mystery. In natural situations we generally expect to find gray squirrels in dense upland forests with well-developed understories. Fox squirrels prefer more open woods–the oak savannas of Illinois would have been perfect for them–with herbaceous rather than woody understories. Reading those habitat descriptions, you would expect that fox squirrels would be the essential city and suburban squirrel. City neighborhoods are places with scattered trees and lots of open ground in the form of lawns and flower beds. So why don’t we have more fox squirrels and fewer gray squirrels?

Around Champaign, the traditional center of abundance for zoology graduate students in this state, the distribution of these two animals has been studied closely, but so far nobody has been able to figure out why they are where they are. In 1966 a student named Sharon Saari surveyed all the towns in Champaign County with populations larger than 150. There are 22 such towns, and 16 of them had only fox squirrels. The only towns with nothing but gray squirrels were Champaign and Urbana, the cities on the plain. Humans regard them as two towns, but squirrels show more sense and think of them as one.

Three towns had both fox and gray squirrels, and in two of these towns both species could be found in the same tree at the same time. In the third town, Ivesdale, the gray squirrels were in the heart of town, while the fox squirrels were in the woodlot out at the edge.

We might conclude from all this that gray squirrels are, for whatever reasons, better adapted to life in the city, but we don’t know what it is that keeps the fox squirrels bottled up in our larger parks. Is it something about the habitat, or is it something about their competitive relations with gray squirrels? We need somebody to spend long hours in Horner Park and around the bird sanctuary unraveling this mystery.

If you are in Lincoln Park or along the lake anywhere between Lincoln Park and Zion, you might keep an eye out for black squirrels. My only sighting in Chicago of one of these animals was on Stratford Place in the block between the Outer Drive and Broadway.

It was an absolutely, totally black squirrel, a deep inky black without a white hair on it. It looked like it had been dragged through a coal bin.

The animal was a melanistic gray squirrel, a genotype that turns up in various places and may be spreading. In Illinois most of our melanistic squirrels are on the North Shore, although there are a few along the Rock River.

Gray squirrels do wander some. In fact, there are historical accounts of large-scale migrations. Robert Kennicott, writing in 1857, says that “immense numbers congregate in autumn, and move off together, continuing their progress in the same general direction, whatever it may be, not even turning aside for large streams.” Unfortunately, that is one of those North American wildlife spectacles we were born too late to witness.