The gray squirrel (Sciurus carolinensis) has been a part of Massachusetts forests for thousands of years. Native Indians and early settlers used the squirrel for food and its fur. In turn, the squirrel raided cornfields and other farm crops during periods of extremely high squirrel populations when thousands of squirrels would leave their normal range and emigrate many miles in search of a new home. The mechanism controlling mass movements of squirrels is still not well understood.

History records that in 1934, two 50-man teams participated in a hunt to reduce the impact of a squirrel population eruption. The two top hunters shot 783 and 900 squirrels, respectively. The days of such hunts are gone forever, but so are the vast forests that could support such high populations, though squirrel populations still periodically erupt and many emigrate from overpopulated areas. Today, increases in road-killed squirrels during late summer and early fall may be a good signal of a peak.

Female squirrels, called sows, may produce 2 litters of young per year if food is plentiful and other factors are not too adverse. Litter size can vary from 1 to 6 young, but is usually between 2 and 4. The two major breeding periods are December through January and May through June, with some minor amount of breeding occurring through the year. Though some juveniles produced from the winter breeding season may produce a litter their first summers, usually only yearlings and adults breed.

Male squirrels, known as boars, carry their testicles inside their bodies during the non-breeding periods. This accounts for the notion that some hunters have about some young boars being clipped by other dominant squirrels to prevent over-population. When the breeding season approaches, the boar’s testicles enlarge and become exposed. Several males usually locate a sow just prior to her entering breeding condition, when she emits a scent that attracts males. Males may chase a sow for several days before she is ready to be bred. The dominant male of the area will usually exert himself over the other males, and gradually the less dominant boars will leave in search of other prospects. Both the sow and her chosen mate will go through a brief courtship until she enters estrus.

Pregnancy lasts about 44 days. First litters are born in February of the winter period, and June of the summer breeding period. As stated previously, the number of young per litter depends on numerous factors, but the average litter is about 3.

The hairless babies weigh about 1/2 an ounce, roughly the weight of a 50-cent piece. At birth, the squirrel’s eyes and ears are closed and no teeth have erupted through the gums. After 3 weeks, the ears of the young squirrel open, followed by the eyes at 5 weeks. From the fifth though the ninth or tenth week, the young squirrel grows about an inch a week. Weaning of the young usually begins in the seventh week. Tooth development, which began in the third week, has progressed and by the seventh week some buds and leaves near the nest can be eaten. However, the bulk of the diet is still milk.

During the eighth week, the young become more active outside the nest and begin to eat more and more solid foods. The first few trips into the outside world provide the young kittens opportunity to chew on various solid matter and learn what is edible. They also begin to become skilled at following mother through the treetops that will someday serve them as protection from predators.

Squirrels use both tree cavities and leaf nests. Young squirrels of both sexes practice building lodging facilities, with varying results in quality of workmanship. Nests often become infested with a high number of fleas, mites, ticks and other insects. However, other insects, including several kinds of beetles, help rid the nests of these biting pests.

Females mated during the summer may return to the den tree cavity where they reared their winter litters, but they often seek new quarters in order to get away from the previous litter. While most litters are reared in tree cavities, some summer litters are reared in leaf nests. Leaf nests are safer during the summer period than during winter because the surrounding foliage protects the nest from view.

Summer-born litters become active outside their nests by September or October. Because the autumn season provides a longer break between breeding periods, the family group will likely stay together somewhat longer before the adult female leaves or drives her young away. Whereas May-bred sows may leave their February-born litters at 8 to 14 weeks after birth, the July-born young will probably stay with the sow into December. The average litter size is about 3 young in the summer and 2.5 young for winter litters.

Scabies or mange, caused by the scabies mite, can be fatal to squirrels. If squirrels scratch themselves until their bodies are bloody and hairless, they become weakened and susceptible to predation, secondary infections and the effects of weather extremes.

Squirrels in Urban Areas
In some cases, squirrels in urban areas may become so abundant and overpopulated that they begin to cause damage to trees, shrubs, ornamental plantings, gardens, and structures.