The United States hosts at least 12 species of tree squirrels, 23 species of ground squirrels, and 2 types of flying squirrels. They are found throughout the country and are classified as game animals in some states. Mostly, though, squirrels are unprotected and considered little more than nuisances.
Tree squirrels generally grow between 1 and 2 feet long including their large, bushy tails. Their coarse, grizzled coats are often shades of gray, brown, or black, and they have lightly colored underbellies. Ground squirrels have shorter tails, short fur, and are generally smaller. They range in appearance from reddish-gray to smoky brown and may have black spots or stripes. Flying squirrels are the smallest of all three types and have webs of skin between their fore and hind limbs used for gliding. Typically light brown or gray, flying squirrels are primarily nocturnal and have large eyes to help them see at night.
Species of tree squirrels prefer to live among hardwood or coniferous forests where ground cover is plentiful. Their nests are usually made of twigs and leaves. Ground squirrels thrive in open grasslands where they dig deep, extensive burrows with multiple exits. Flying squirrels make their homes in the crevices of trees or abandoned bird's nests. The small, nocturnal creatures prefer to live in wooded areas where they can glide from treetop to treetop without having to descend to the ground.
Squirrels are common sights in backyards and public parks. As opportunistic omnivores, they feed off human food and garbage whenever possible. Tree squirrels and flying squirrels have been known to enter unsecured attic spaces to nest, and ground squirrels often burrow into yards or farmers' fields in search of food.
Taking advantage of any available food sources, squirrels damage yards and fields by digging for nuts and insects. When ground squirrels burrow near manmade embankments or dams, they weaken the structural integrity and cause collapses. Flying squirrels and tree squirrels damage homes by gnawing or tearing at insulation and electrical wires on roofs and in attics. They can also cause outages when they run across power lines.