Tree squirrels include over a hundred species that are found on all continents except Antarctica, and are the members of the squirrel family (Sciuridae) most commonly referred to as "squirrels". They do not form a single natural, or monophyletic, group, but instead are related to the various other animals in the squirrel family, including ground squirrels, flying squirrels, marmots, and chipmunks. The defining characteristic that is used to determine which of the various species of Sciuridae are tree squirrels is therefore not so dependent on their physiology, but their habitat. Tree squirrels live mostly among trees, as opposed to other squirrels that live in burrows in the ground or among rocks. However, there is one exception to this rule, as physiological distinction does make a difference in regard to flying squirrels, who also make their home in trees, but have unique physical characteristics that separate them from their tree squirrel cousins (specifically, special flaps of skin that act as glider wings, allowing them to "fly").
The most well known genus of tree squirrels is Sciurus, which includes the Eastern gray squirrel of North America (and which was introduced]when?[ to Great Britain]citation needed[), the red squirrel of Eurasia, and the North American fox squirrel, among many others. Since many tree squirrel species have readily adapted to human-altered environments (including intensely cultivated farms and urban cities), and because they are mostly diurnal (active during the daytime), when most people are outdoors to see them, they are perhaps the most familiar members of the rodent family to most humans. In some larger cities, they are often the only wild animals (not counting birds) that most people ever see.
A bird nest is the spot in which a bird lays and incubates its eggs and raises its young. Although the term popularly refers to a specific structure made by the bird itself—such as the grassy cup nest of the American Robin or Eurasian Blackbird, or the elaborately woven hanging nest of the Montezuma Oropendola or the Village Weaver—that is too restrictive a definition. For some species, a nest is simply a shallow depression made in sand; for others, it is the knot-hole left by a broken branch, a burrow dug into the ground, a chamber drilled into a tree, an enormous rotting pile of vegetation and earth, a shelf made of dried saliva or a mud dome with an entrance tunnel. The smallest bird nests are those of some hummingbirds, tiny cups which can be a mere 2 cm (0.79 in) across and 2–3 cm (0.79–1.2 in) high. At the other extreme, some nest mounds built by the Dusky Scrubfowl measure more than 11 m (36 ft) in diameter and stand nearly 5 m (16 ft) tall.
Not all bird species build nests. Some species lay their eggs directly on the ground or rocky ledges, while brood parasites lay theirs in the nests of other birds, letting unwitting "foster parents" do all the work of rearing the young. Although nests are primarily used for breeding, they may also be reused in the non-breeding season for roosting and some species build special dormitory nests or roost nests (or winter-nest) that are used only for roosting. Most birds build a new nest each year, though some refurbish their old nests. The large eyries (or aeries) of some eagles are platform nests that have been used and refurbished for several years.
The western gray squirrel (Sciurus griseus) is an arboreal rodent found along the western coast of the United States and Canada.
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).
The Northern flying squirrel (Glaucomys sabrinus) is one of two species of the genus Glaucomys, the only flying squirrels found in North America (the other is the somewhat smaller Southern flying squirrel, G. volans). Unlike most members of their family, flying squirrels are strictly nocturnal. The Northern flying squirrel is found in coniferous and mixed forests across the top of North America, from Alaska to Nova Scotia, south to the mountains of North Carolina and west to California. Populations from the Pacific Coast of the United States are genetically distinct from those of G. sabrinus found elsewhere in North America, although they are considered to belong to the same species. Two subspecies are found in the southern Appalachians, the Carolina Northern flying squirrel, G. s. coloratus, and the Virginia Northern flying squirrel G. s. fuscus, both of which are endangered, although the Virginia subspecies has recovered enough that it was delisted in August 2008. The U.S. Fish and Wildlife Service put the flying squirrel back under protection on June 6, 2011.
The nocturnal, arboreal rodents have thick light brown or cinnamon fur on their upper body and greyish on the flanks and whitish underneath. They have large eyes and a flat tail. They can also be identified by their long whiskers, common to nocturnal mammals. The adult northern flying squirrel measures from 25 to 37 cm long, and their weight can range from 110 to 230 grams.