Eastern Grey Squirrels (Sciurus carolinensis) are one of the most common denizens in our urban and suburban landscapes (Russell Link, Living with Tree Squirrels. Washington Department of Fish and Wildlife, Olympia, Washington, 2004). Most are rusty grey with whitish belly, but I regularly have black squirrels in my yard, too. Their original habitat is the mixed eastern deciduous forest, which is what Evanston originally was, though now the town's vegetation is infiltrated with many foreign ornamental plants. Squirrel home ranges vary from several acres in forest and a half acre in suburbs. They live in tree hollows and occasionally inside structures, if they can get in, as happened once in my attic, as I describe, below.
A lot of people give nut or seed treats to the squirrels. I do put food out for them to keep them away from the bird feeder and from my vegetable and fruit plantings. That strategy seems to work except that they sometimes bury their seeds in my flowerpots, disturbing what I'd planted there. I can see them carrying off some piece of food like a nut in their mouth to a spot where they dig furiously, deposit the food item, then cover it up neatly and pat the surface flat with their handlike front paws. Supposedly they remember where they bury things and dig them out when they need food. In any case, the sunflower seeds they plant throughout my yard yield lots of large, tall yellow flowers in late summer, so I never have to plant sunflowers.
My bird feeder has defenses against squirrels that work if I keep them adjusted the right way. It has bars for birds to stand on that descend when too great a weight is put upon them, as when a squirrel or crow steps on the bar. The feeder has three possible weight settings that you choose by hooking levers over particular stops. You choose the stop based on the weight of the deserving birds and the feathered or furry thieves that might come to your feeder.
I notice that the squirrels' food is very seasonal. In spring, they busily nip off the young buds and leaf shoots, resulting in the pruning of certain trees, especially maples. I find on the lawn many bunches of young leaves they've bitten off and dropped. In summer, they eat certain early fruits and seeds, though there are much more of those in fall. To eat those most conveniently, they bite off the ends of branches and drop them, harvesting the fruits or seeds on the ground. Both the squirrels and birds spend a lot of time in my hawthorn and crab-apple trees in the fall. In the depth of winter, they have the things they've stored away, and they also can eat the dried-out fruits that some shrubs and trees hold there on their branches.
Squirrels are very territorial and intolerant of other squirrels once grown. They chase each other to keep their territory for themselves, or to try to, and they mark with urine, seemingly to warn off others and claim a food gathering spot for themselves. I often find a small urine spot on my back steps near the compost pile, where edibles are known to land. The small, young squirrels from the year's litter tend to hang out in the most dangerous places, such as the parkway trees, since they they often have to cross the street to get between them. The older yearling squirrels tend to hog the safer places that have better access to bird feeders, compost heaps, and seed-bearing garden plants.
I find squirrels to be smart and socially adept. They know who different humans are, distinguishing those who are tolerant of squirrels and might have a bit of food for them from those who think of them as vermin. They also understand that a screen door between them and a cat means the cat can't get at them, so they need not be wary. The mother squirrels are the most tame with humans and the most assertive. They come right up to the kitchen door to demand their seeds. I don't hand-feed the squirrels, though, to prevent them becoming unwary.
Once one of the black-furred mothers took up in the attic to make her nest for her young. (It's actually not a real attic since you can't stand up in it.) I had the animal removed by a "humane" animal service because squirrels have good memories and will eat their way back in the next year if you close the original hole. (The guy from the animal service put his foot through the trap door to my "attic" space, leaving a hole that then caused ice dams because it let warm air from the house up under the roof. The warm air made the ice and snow on the roof melt and the resulting water, dammed above the frozen eaves, seeped into the house, causing stains and plaster damage. Insurance paid for the repairs but then cancelled my contract because of them!) That squirrel mother looked at me reproachfully from the humane trap as she was carried off to the forest reserve. At least I hope that's where they took her. I've since learned from wildlife manuals that translocating squirrels is not necessarily a good idea, because most die. However, the manner of trapping may be the most important cause of mortality (Curtis O. Bosson, Rupert Palme, and Rudy Boonstra, Assessing the impact of live-capture, confinement, and translocation on stress and fate in eastern grey squirrels, Journal of Mammalogy 94(6): 1401-1411, 2013). Trapping can stress the animals - leaving traps in the sun, for example -, and in the new place, they have to learn the new landscape and may have serious competitors among the better acclimated squirrels already there and also new predators.
Another black-furred mother squirrel lived in the yard for at least 5 years. She raised a succession of plump grey young who were not at all respectful to her when grown. They would chase her all around the yard and displace her from good feeding spots. She was very tame and would forage for slugs and bugs at the edges of the grass, where, if I happened to be weeding or planting, she would walk right over my feet or hands if they were in her way. Her tiny hands felt cool, damp, and a bit prickly from her sharp little nails. When old, she was very thin, her limbs creaky, and her fur ratty with bare patches, but she survived a long time that way, then eventually disappeared.
Squirrels preen off their old fur once a year, then grow a new coat. Squirrels in our yards also have illnesses and disabilities. Sometimes I'll see ones who'd lost part of their tail or had a lesion on it. Through the years I've seen many neurologically disabled ones. They do not lose their intelligence, for they continue to beg food and wait at times and places they remember food is likely to be forthcoming. But their skeletal muscles seem to turn faulty. They tremble. They cannot hold their heads up well nor turn them left or right. They become clumsy and often fall off branches. Despite their disabilities, these squirrels are usually fat, perhaps because they are not as active as healthy ones but still have a food source, and they do live well into adulthood. I don't know if they may have developed something like mad cow disease, a disorder deer get in conditions of crowding. Such neurologically disabled squirrels have been studied and found sometimes to have parasites, been poisoned, or been injured in an accident. The disease may come when populations are too crowded, as they can be in our suburbs. It could be possible that the ill squirrels in my yard have eaten something poisonous, like the lead edging of my roof vent pipes, which my roofer says has been chewed off. (Squirrels are not documented as spreading diseases to humans with the exception of rabies in some states (Link 2004). Because of the possibility of rabies, it's best not to handle squirrels, especially those that appear to be sick. If you find a baby squirrel on the ground, keep other animals away so the mother can retrieve it. If she doesn't, call the licensed Wildlife Rehabilitator in your area to pick it up. Any animal shelter can provide the contact number.)
Squirrels have quite a range of vocalizations for different occasions. The most common is a loud, harsh chattering accompanied by quick tail flicks, done usually by several animals in concert. This most often is a response to the intrusion of a cat, dog, or coyote into the yard, though I've heard this call when the squirrels saw a rabbit or when I surprised them by appearing suddenly. Another common call is more plaintive, like wailing. I hear it when the young squirrels born in a year are chasing each other around in the trees. The one being chased will make these wailing calls. The neurologically disabled squirrels I mentioned above have faint, high, querulous calls unlike the usual ones, perhaps because their vocal chords, a kind of muscle, don't work right any more.