In some places, this species has also been known as the silver-gray squirrel, the California gray squirrel, the Oregon gray squirrel, the Columbian gray squirrel and the banner-tail. There are three geographical subspecies: Sciurus griseus griseus (central Washington to the western Sierra Nevada in central California); S. g. nigripes (from south of San Francisco Bay to San Luis Obispo County, California; and S. g. anthonyi, which ranges from San Luis Obispo to northern Baja California).
|Western gray squirrel|
|Western gray squirrel's range|
Sciurus griseus is the largest tree squirrel in the Sierra Nevada and Central California range. It has plantigrade, pentadactyl feet with two phalanges. Compared with the eastern gray squirrel S. carolinensis or the fox squirrel S. niger (which have been introduced into its native range), these squirrels are shy, and will generally run up a tree and give a hoarse chirping call when disturbed. Weights vary from about .35 to 1 kilogram (0.77 to 2.20 lb), and length (including tail) from 43 to 61 centimetres (17 to 24 in). It is the largest native tree squirrel in the western coastal United States. Western gray squirrels exhibit a form of coloration known as counter shading. The dorsal fur is a silver gunmetal gray, with pure white on the underside; there may be black flecks in the tail. Ears are large but without tufts. The ears turn reddish-brown at the back in the winter. The tail is long and typically very bushy. Also, it stays in a curved upwards in an "S" shape.
Tree squirrels undergo a complete head-to-tail molt in the spring and a rump-to-head molt in the fall. Tail hair is replaced only in the spring. Nesting mothers will use their tail hair to line birthing nests. Western gray squirrels eat berries, nuts, a variety of seeds, and the eggs of small birds. The dental formula for Sciurus griseus is 220.127.116.11 × 2 = 22.
Western gray squirrels reach sexual maturity at 10 to 11 months, and at approximately one year of age, will begin breeding. They mate over an extended period ranging from December through June. Young are born after approximate 44-day gestation period. Juveniles emerge from nests between March and mid-August. Litter sizes range from one to five kits which remain in the nest for a longer period than other squirrels. The kits are relatively slow in development, and will not leave the nest for six months or more, another species disadvantage when in competition with other, more-rapidly fledging squirrels. Young gray squirrels have furled tails which will not reach fullness until adulthood. This is a good indicator of age and maturity. Mother squirrels often seem to be overworked with a stressed appearance, complete with bruised and battered nipples. Mating squirrels can be very physical and will bite and injure each other. Females can be quite territorial, and will chase others away and have fairly violent altercations between themselves.
Western gray squirrels are forest dwellers, and can be found at elevations up to 2,000 m. Time on the ground is spent foraging, but they prefer to travel distances from tree to tree. They are strictly diurnal, and feed mainly on seeds and nuts, particularly pine seeds and acorns, though they will also take berries, fungus and insects. Pine nuts and acorns are considered critical foods because they are very high in oil and moderately high in carbohydrates, which help increase the development of body fat. They feed mostly in trees and on the ground. They generally forage in the morning and late afternoon for acorns, pine nuts, new tree buds, and fruits. They feed on pinecones and many other nuts in preparation for the winter.
When on alert, they will spread their tails lavishly, creating an umbrella effect that shields them and possibly provides cover from overhead predators. They are scatter-hoarders making numerous caches of food when it is abundant, and thus contribute to the seed dispersion of their food trees. Although squirrels show relatively good scent relocation abilities, some food caches are never reclaimed, becoming seedlings in the spring. Though they do not hibernate, they do become less active during the winter. Like many prey animals, they depend on auditory alerts from other squirrels or birds to determine safety. Once an alarm call is transmitted, those present will join in, and the trees become a cacophony of chirping squirrels. Tree squirrels are prey for bobcats, hawks, eagles, mountain lions, coyotes, cats, raccoons and humans.
Squirrel nests are called dreys and can be seen in trees, built from sticks and leaves wrapped with long strands of grass. There are two stick nest types made by the western gray squirrel: the first is a large, round, covered shelter nest for winter use, birthing, and rearing young. The second is more properly termed a "sleeping platform," a base for seasonal or temporary use. Both types are built with sticks and twigs and are lined with leaves, moss, lichens and shredded bark. The birthing nest may be lined with tail hair. The nest may measure 43 to 91 cm (17 to 36 in) by up to 46 cm (18 in) and is usually found in the top third of the tree. Young or traveling squirrels will also "sleep rough" when weather permits, balanced spread-eagled on a tree limb high above the forest floor. This attitude is also adopted for cooling in hot weather, a behavior also observed in raccoons.
The Western Gray Squirrel is an arboreal species, requiring enough tree cover for arboreal travel. It resides in woody areas to build their complex nests. It lives in high and low elevations in California and finds habitats in both walnut trees and black oak trees.
The Western Gray Squirrel has many predators including Red-tailed Hawks (Buteo jamaicensis), Great-horned Owls (Bubo virginianus), eagles (Accipitridae), Bobcats (Lynx rufus), Coyotes (Canis latrans), Cougars (Puma concolor), domestic cats (Felis catus) and dogs (Canis lupus familiaris). However, the predation does not control the squirrel's population density 
The western gray squirrel was listed as a threatened species in Washington state in 1993. Populations of the western gray squirrel have not recovered from past reductions. They are threatened with habitat loss, road-kill mortality and disease. Habitat has been lost due to urbanization, catastrophic wild fires, and areas of forest degraded by fire suppression and overgrazing, allowing the invasion of scotch broom. Notoedric mange, a disease caused by mites, becomes epidemic in western gray squirrel populations and is a major source of mortality. Other species of eastern gray squirrels, fox squirrels, California ground squirrels and wild turkeys are expanding and compete with the western gray.
Listed as extirpated in some California areas, the western gray squirrel in southern California is generally found only in the mountains and surrounding foothill communities. Local rehabilitation experts recount the Eastern Fox Squirrels were released in urban regions of Los Angeles throughout the 20th Century. Fox squirrels (Sciurus niger) were introduced to the Los Angeles area in about 1904. Civil War and Spanish–American War veterans residing at the Sawtelle Veterans Home on Sepulveda and Wilshire Boulevards brought fox squirrels as pets to this site from their homes in the areas surrounding the Mississippi Valley (possibly Tennessee). Other introductions of fox squirrels to the Los Angeles area may have taken place during more recent times but detailed records are not available. These aggressive cousins drove the more reclusive western grays back into the mountains, where competition was not so strong. This non-native species introduction appears to be the largest threat in the southern California area.
Allen's squirrel (Sciurus alleni) is a tree squirrel in the genus Sciurus endemic to northern Mexico. It has no subspecies.Bangs's mountain squirrel
Bangs's mountain squirrel (Syntheosciurus brochus) is a poorly known species of tree squirrel, that only lives in Costa Rica and Panama. It can be found in mountain rain forests at an altitude between 1,900 and 2,600 metres (6,200 and 8,500 ft), and lives mainly in the tree tops, but sometimes on the forest floor as well. One of its habitats is at the summit of the Poás Volcano in Costa Rica, in a Clusia forest that is almost inaccessible to humans.Bolivian squirrel
The Bolivian squirrel (Sciurus ignitus) is a tree squirrel that is endemic to South America. Little is known of the species, which may represent a species complex.Brazilian squirrel
The Brazilian squirrel (or Guianan squirrel) (Sciurus aestuans) is a tree squirrel in the genus Sciurus endemic to South America. It is found in South-eastern Colombia, Brazil, Guyana, French Guiana, Suriname and Venezuela.
It is a dark brown squirrel that feeds mainly on fruits and nuts, but can also prey on eggs and the young of birds.Collie's squirrel
Collie's squirrel (Sciurus colliaei) is a tree squirrel in the genus Sciurus endemic to Mexico.Eastern gray squirrel
Sciurus carolinensis, common name eastern gray squirrel or grey squirrel depending on region, is a tree squirrel in the genus Sciurus. It is native to eastern North America, where it is the most prodigious and ecologically essential natural forest regenerator. Widely introduced to certain places around the world, the eastern gray squirrel in Europe, in particular, is regarded as an invasive species.Gray squirrel
Gray squirrel may refer to several species of squirrel indigenous to North America:
The eastern gray squirrel (Sciurus carolinensis), from the eastern United States and southeastern Canada; introduced into Britain, Ireland, western North America, Italy, and South Africa
The western gray squirrel (Sciurus griseus), from the western United States
The Arizona gray squirrel (Sciurus arizonensis), from the southwestern United States and adjacent Mexico
The Mexican gray squirrel (Sciurus aureogaster), from southern Mexico and Guatemala; introduced into the Florida KeysJapanese squirrel
The Japanese squirrel (Sciurus lis) is a tree squirrel in the genus Sciurus endemic to Japan. The Japanese squirrel's range includes the islands of Honshū, Shikoku, and Kyūshū. Recently, populations on south-western Honshū and Shikoku decreased, and those on Kyūshū disappeared. One of the factors affecting the local extinction of this species seems to be forest fragmentation by humans.Northern Amazon red squirrel
The northern Amazon red squirrel, Sciurus igniventris, is a squirrel species from South America. It occurs in Brazil, Colombia, Ecuador, Peru and Venezuela.Peters's squirrel
Peters's squirrel (Sciurus oculatus) is a tree squirrel in the genus Sciurus endemic to Mexico. It was first described by the German naturalist and explorer Wilhelm Peters in 1863. Three subspecies are recognised. It is a common species, and the IUCN has rated its conservation status as being of "least concern".Pine squirrel
Pine squirrels are squirrels of the genus Tamiasciurus, in the Sciurini tribe, of the large family Sciuridae.Santander dwarf squirrel
The Santander dwarf squirrel Microsciurus santanderensis is a small tree squirrel endemic to Colombia.Sciurinae
Sciurinae is a subfamily of squirrels in the (family Sciuridae), uniting the flying squirrels with certain related tree squirrels. Older sources place the flying squirrels in a separate subfamily (Pteromyinae) and unite all remaining sciurids into the subfamily Sciurinae, but this has been strongly refuted by genetic studies.Sciurus
The genus Sciurus contains most of the common, bushy-tailed squirrels in North America, Europe, temperate Asia, Central America and South America.Sierra Nevada lower montane forest
The Sierra Nevada lower montane forest is a plant community along a strip along the western and eastern edges of the Sierra Nevada mountain range in California. This zone is also known as a yellow pine forest.
Moving southward, the elevation range of the lower montane forest grows. Its elevation range at the northern end of the Sierra is from 1,200–5,500 feet (370–1,680 m). In the central Sierra, its elevation range is from 3,000–7,000 feet (910–2,130 m). In the southern end of the Sierra, its range is from 2,500–9,000 feet (760–2,740 m),
At the lower elevation of this forest, the hot, dry summers and cool, moist winters of the Mediterranean climate give rise to the lower montane forest zone. The accumulation of several feet of snow during the winter is not uncommon and can stay on the ground for several months. The diversity of tree species found in this zone make this a beautiful and interesting forest to explore. The indicator species for the lower montane forest are the ponderosa pine and the Jeffrey pine: the ponderosa pine generally occurs on the west side of the Sierra, while the Jeffrey pine occurs on the east. The lower montane forests also include trees such as California black oak, sugar pine, incense-cedar, and white fir. The giant sequoia groves of the Sierra Nevada are also found within this biotic zone. Animals that may be found in this zone include the dark-eyed junco, mountain chickadee, western gray squirrel, mule deer, and American black bear. The lower montane forest can be seen in Yosemite Valley and along the Wawona, Hetch Hetchy, and Big Oak Flat Roads.Squirrel
Squirrels are members of the family Sciuridae, a family that includes small or medium-size rodents. The squirrel family includes tree squirrels, ground squirrels, chipmunks, marmots (including woodchucks), flying squirrels, and prairie dogs amongst other rodents. Squirrels are indigenous to the Americas, Eurasia, and Africa, and were introduced by humans to Australia. The earliest known squirrels date from the Eocene period and are most closely related to the mountain beaver and to the dormouse among other living rodent families.Yellow-throated squirrel
The yellow-throated squirrel (Sciurus gilvigularis) is a tree squirrel in the genus Sciurus endemic to South America. It is found in Brazil, Guyana and Venezuela.