Fox and gray squirrels first breed when they are about a year old. They breed in mid-December or early January, and a small percentage breeds again in June. Young squirrels may breed only once in their first year. The gestation period is 42 to 45 days.
During the breeding season, noisy mating chases take place when 1 or more males pursue a female through the trees. Tree squirrels have about 3 young per litter. At birth, they are hairless, blind, and their ears are closed. Young weigh about ½ ounce at birth and 3 to 4 ounces at 5 weeks. At weaning they are about half of their adult weight. Young begin to explore outside the nest about the time they are weaned at 10 to 12 weeks. Typically, about half of the squirrels in a population die each year. In the wild, squirrels over 4 years old are rare while individuals may live 10 years in captivity.
The biology of other North American squirrels is similar to that of fox and gray squirrels, although most other species have just 1 breeding season per year. Flying squirrels are active at night. All other species are active primarily during the day.
Squirrels are cavity dwellers, preferring hollow trees and buildings (Figure 4a). Large leafy nests (Figure 4b) also are constructed particularly in the summer, as they are cooler.
Tree squirrels nest in tree cavities, human-made squirrel boxes, or leaf nests. Leaf nests, called dreys, are constructed with a frame of sticks filled with dry leaves and lined with leaves, strips of bark, corn husks, or other materials. Survival of young in cavities is higher than in leaf nests, making cavities preferred sites for nests.
Individual home ranges vary from 1 to 100 acres, depending on the season and availability of food. Squirrels move within their range according to the availability of food. They often seek mast-bearing forests and corn fields in the fall and tender buds of maple trees are favored in the spring. During fall, squirrels may travel 50 miles or more in search of better habitat. Populations of squirrels fluctuate regularly. When population numbers are high, squirrels, especially gray squirrels, may go on mass emigrations where many individuals die.
The ecological overlap of fox and gray squirrels varies somewhat by region. In the Southeastern US, fox squirrels often are associated with mature pine forests of longleaf and loblolly, while gray squirrels occupy any habitat with sufficient mast-producing hardwoods. In the Midwest, fox squirrels tend to be more associated with agricultural woodlots, while gray squirrels tend to use larger forest blocks. In other regions, one species may tend to occupy bottomland forests and the other upland areas. Gray squirrels often are found in great numbers in cities, especially in and around parks. The presence of mature hardwoods often is important to both species. Flying squirrels, being more arboreal (tree-dwelling), are restricted to areas of large, mature hardwoods.
It is important to distinguish the types of food storage used by squirrels. Large squirrels, such as gray and fox squirrels, scatter cache, which means they store individual acorns or other seeds (mast) in different areas around their home range. Smaller squirrels, such as red squirrels, store food in 1 place. It is not uncommon to find trash-bag-sized piles of pine cones inside attics or gutters, brought there by red squirrels.
Fox and gray squirrels have similar food habits. They eat a variety of native foods and adapt quickly to unusual sources of food. Typically, they feed on mast (wild tree fruits and nuts) in fall and early winter. Acorns, hickory nuts, walnuts, and Osage orange fruits are favorite fall foods. Fox squirrels feed heavily on pine cones; gray squirrels will as well in spring. Nuts often are cached for later use. In late winter and early spring, both species prefer tree buds. In summer, they eat fruits, berries, and succulent plant materials. Fungi, corn, and cultivated fruits are taken when available. Squirrels chew bark from a variety of trees.
The food habits of flying squirrels generally are similar to those of other squirrels, though they are the most carnivorous of all tree squirrels. They eat bird eggs and nestlings, insects, and other animal matter when available. Flying squirrels often occupy bird houses.