is almost always at least above the ground, and flying squirrels build much higher. Sometimes squirrels build their own dreys and sometimes they occupy a vacant drey that was previously constructed by another tree squirrel, often of a different species. Dreys must protect against the environment, and require constant upkeep to remain water- and predator-resistant. Squirrels often build more than one in a season, as reserve nests, lest the primary drey be disturbed by predators or overrun by fleas or lice. Some dreys have been observed in use for more than a decade by multiple generations of squirrels, although the average drey may be used only a year or two before being abandoned. If used repeatedly, squirrels must constantly maintain their drey, replenishing twigs and leaves as necessary. Remnants of an abandoned nest may be visible for years. Occupation Male and female squirrels may share the same nest for short times during a breeding season, and during cold winter spells squirrels may share a drey to stay warm. However, females nest alone when pregnant. In North America, squirrels produce broods of about three "pups" twice a year. (After leaving the drey, a young squirrel is termed a "juvenile" for its first year of life.) The June broods are sometimes born in dreys, but January broods are usually born and raised in tree cavities, which are much safer. Drey broods are about 40% less likely to survive than tree cavity broods, so long as the cavity entrance hole is no wider than about 15 cm, which can keep out hungry raccoons. References Shelters built or used by animals Squirrels