There are an astonishing number of squirrels worldwide, and many are strikingly attractive animals. In North America, there are several species that are regarded as "tree" squirrels, contrasting with an even greater variety of "ground" squirrels. Tree squirrels are found throughout most of the United States—except the treeless Great Plains and Great Basin areas—and into southern Canada.

The fox squirrel (Sciurus niger) and eastern (S. carolinensis) and western (S. griseus) gray squirrels are usually the species involved in homeowner conflicts. Of course, squirrels also have many engaging and fascinating behavioral traits that endear them to us, and make squirrel watching an educational and enriching experience.

The eastern gray squirrel measures 8 to 10 inches and has a bushy tail that's almost as long. This squirrel can be highly varied in coloration, ranging from a rufous tint to almost pure white, or even all dark or black. The western gray squirrel is a little longer and heavier than his eastern cousin, while the fox squirrel is the biggest of all—as long as 15 inches with a tail almost as long, and weighing up to three pounds.

Fox and gray squirrels traditionally depend on trees, especially oak trees, as places to bear and raise young, take shelter from the weather, find food, and escape from predators. Squirrels use two basic types of natural dens—tree cavities and leaf nests. But having seamlessly adapted to life in urban and suburban areas, squirrels also take liberal advantage of shelter provided by humans in attics, crawl spaces along the upper floors of buildings, pilings around marinas, and nest boxes intended for various species of birds.

The squirrel diet varies with the seasons and the availability of plant material (such as nuts, fruits, flowers, and plant buds) that almost entirely makes up the animal's larder. Squirrels also occasionally eat bird eggs or nestlings, and may even pounce on small birds at feeders—much to the dismay of human witnesses who assume that squirrels are passive vegetarians.

Eastern gray and fox squirrels usually have two litters a year, with young born between February and April, then again in August or September. The western gray squirrel has only one breeding season a year, and young are born between February and June. Young are born naked and helpless, and don't venture out of the nest for about seven or eight weeks.

Squirrels are fascinating animals to watch, photograph, and study. They make themselves available for observation in ways that few other animals do, and they have a rich and complex repertoire of behaviors. For those willing to patiently watch and learn, squirrels have a lot to offer.