Gray squirrels thrive in city parks and wild oak-hickory forests. Many people enjoy feeding them in the backyard, but just as many wish for truly squirrel-proof bird feeders. Despite their familiarity, gray squirrels, and all tree squirrels for that matter, are perfectly adapted to an arboreal lifestyle.

Living in the tree tops shapes the form and function of gray squirrels in many ways. Their primary food source, nuts, is available for only a few months each year. They are exposed to extremes of wind, rain, snow and specialized predators. And expert gymnastic skills are essential for moving through the tree tops.

Unlike other members of the squirrel family such as chipmunks and groundhogs, gray squirrels do not hibernate. They cope with wintery conditions and food shortages in other ways. During the fall they feast on hard mast -- acorns, hickory nuts, walnuts and beech nuts. At other times of the year, they subsist on fruits, berries, buds, mushrooms and an occasional bird egg or nestling.

Gray squirrels eat madly during the fall and increase their body mass by about 10 percent. Thus, they enter winter fat and sassy, able to eat less and reduce their outside activities. This minimizes their exposure to the elements and predators. And in between their frenzied fall meals, they collect and store large quantities of food that provides food all winter long.

After food, nests are a gray squirrel's most valuable resource. During mild weather, they construct leaf nests high in tree tops, and females often use leaf nests as nurseries. But the best and most secure nests are tree cavities. Hole size excludes larger predators, and den trees provide excellent shelter from the elements. Gray squirrels will also use nest boxes, but if given a choice they prefer natural nest sites.

Finally, sharp curved claws and an anatomical adaptation worthy of a contortionist allow tree squirrels to move effortlessly from massive tree trunks to dainty branches. Watch a squirrel's hind feet as it descends a tree. They rotate their ankles 180 degrees so their rear feet face backwards. This gives squirrels incredible strength and dexterity.