The squirrel is a small or medium-sized rodent of the family Sciuridae. In the English-speaking world, it commonly refers to members of this family's genera Sciurus and Tamiasciurus, which are tree squirrels that have large bushy tails, and are indigenous to Europe (but not Ireland), Asia and the Americas. Similar genera are found in Africa. The Sciuridae family also include flying squirrels, as well as ground squirrels such as the chipmunks, prairie dogs, and woodchucks. Members of the family Anomaluridae are sometimes misleadingly referred to as "scaly-tailed flying squirrels" although they are not closely related to the true squirrels.
The word squirrel, first attested in 1327, comes via Anglo-Norman esquirel from the Old French escurel, the reflex of a Latin word which was itself borrowed from Greek. The native Old English word, acweorna, only survived into Middle English (as aquerna) before being replaced.
Common squirrels include the Fox Squirrel (S. niger); the Western Gray Squirrel (S. griseus); the Douglas Squirrel (Tamiasciurus douglasii); the American Red Squirrel T. hudsonicus; and the Eastern Grey Squirrel (S. carolinensis), of which the "Black Squirrel" is a variant.
Unlike rabbits or deer, squirrels cannot digest cellulose and must rely on foods rich in protein, carbohydrates, and fat. Early spring is the hardest time of year for squirrels, since buried nuts begin to sprout and are no longer available for the squirrel to eat, and new food sources have not become available yet. During these times squirrels rely heavily on the buds of trees, in particular, those of the Silver Maple. Squirrels are omnivores; they eat a wide variety of plant food, including nuts, seeds, conifer cones, fruits, fungi, and green vegetation, and eat insects. Ground and tree squirrels are typically diurnal, while flying squirrels tend to be nocturnal -- except for lactating flying squirrels and their offspring, who have a period of diurnality during the summer.
Predatory behavior by various species of ground squirrels, particularly the thirteen-lined ground squirrel, has been noted. Bailey (1923), for example, observed a thirteen-lined ground squirrel preying upon a young chicken. Wistrand (1972) reported seeing this same species eating a freshly-killed snake. Whitaker (1972) examined the stomachs of 139 thirteen-lined ground squirrels, and found bird flesh in four of the specimens and the remains of a short-tailed shrew in one; Bradley (1968), examining white-tailed antelope squirrels' stomachs, found at least 10% of his 609 specimens' stomachs contained some type of vertebrate � mostly lizards and rodents. Morgart (1985) observed a white-tailed antelope squirrel capturing and eating a silky pocket mouse.