Fox and gray squirrels breed when they are 1 year old. They breed in mid-December or early January and 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 one or more males pursue a female through the trees.
They nest in tree cavities, human-made squirrel boxes, or in leaf nests. Leaf nests 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. Cavities are the preferred nest sites.
About 3 young comprise a litter. At birth they are hairless, blind, and their ears are closed. Newborns weigh about 1/2 ounce (14 g) at birth and 3 to 4 ounces (84 to 112 g) at 5 weeks. Young begin to explore outside the nest about the time they are weaned at 10 to 12 weeks. At weaning they are about half of their adult weight.
Home range size depends on the season and availability of food. It may vary from 1 to 100 acres (0.4 to 40 ha). Squirrels move within their range according to availability of food. They often seek mast-bearing forests in fall and favor tender buds in elm and maple forests in the spring.
During fall, squirrels may travel 50 miles (80 km) or more in search of better habitat. Squirrel populations periodically rise and fall. During periods of high populations, squirrels--especially gray squirrels--may go on mass emigrations. At such times many animals die.
Fox and gray squirrels are vulnerable to numerous parasites and diseases. Ticks, mange mites, fleas, and internal parasites are common. Squirrel hunters often notice bot fly larvae (called “wolves” or “warbles”) protruding from the skin. These fly larvae do not impair the quality of the meat for eating. Squirrels are a food source for hawks, owls, snakes, and several mammalian predators. Predation seems to have little effect on squirrel populations.
Typically about half the squirrels in a population die each year. In the wild, squirrels over 4 years old are rare, while in captivity individuals may live 10 years or more.
The biology of other North American squirrels has much in common with that of fox and gray squirrels, although most other species have one breeding season per year. Flying squirrels are unique in that they are active at night. All other species are active during the day.