Douglas squirrels range from southern British Columbia southward to Lower California. They are confined to the Pacific Coastal region. Although closely related to Tamiasciurus hudsonicus, no intergradation with that species has been found.
The rich coloration of the Douglas squirrel matches the background of the humid forests it inhabits. It occurs at sea level along the ocean coast, the shores of Puget Sound and on some islands in Puget Sound. It ranges through the Transition and Canadian life-zones well into the Hudsonian.
It makes its home in holes in trees or in nests constructed of conifer twigs, needles and bark. Old nests of birds may be modified and taken over, but most outside nests seem to have been made entirely by the squirrel. Nests are usually placed near the top of denser fir and cedar trees. Nests are hemispherical in shape, 12 inches or more in diameter, and open on top. The central cup, 4 inches in diameter, is lined with strips of inner bark of red cedar coiled but unshredded. Nests in holes are usually some distance from the ground, but the entrances to some are at ground level.
The habits of the Douglas squirrels vary with the season. In spring they are shy. They rarely call and are skillful in keeping branches or tree trunks between themselves and persons. In June and July they become bolder, calling more frequently and moving about more. In September and October they become exceptionally bold. They call almost incessantly from late morning until early afternoon, and spend considerable time on the ground.
The call notes of the Douglas squirrel are similar to those of the red squirrel but are softer, more slurred. The common call is the long, trilling "burr," loud at first and becoming gradually softer. On hot afternoons in late summer a barking "pauf" note is given. This sound is repeated several times at intervals of a minute or so. It has remarkable carrying power. The danger signal is a loud, explosive "pe-ee." Another common call is a short barking or chirping "bauf" with a musical, questioning sound. This is repeated at intervals of a few seconds when the presence of danger is suspected. At times it, rather than the loud danger signal, is given also when a person is sighted. A low chirping note is often given while the squirrel is busily at work. This is made while the squirrel is moving about. Other calls described are made from a standing or sitting position and are accompanied by a jerk or a flick of the tail.
The Douglas squirrel eats seeds of the Douglas fir, seeds of maple, hazelnuts, dogwood berries and mushrooms. The berries of forest shrubs, such as the red huckleberry, salal and Oregon grape, may also be eaten.
Two broods of young may be born in a year. The first is born in early June. Embryos found from June 11 to 28 varied in number from 6 to 8. A nursing female was taken as early as June 10, 1938, and one was taken as late as October 10, 1938.
Douglas squirrels commonly have a few fleas and 2 to 5 ticks at the bases of the ears. No lice, cases of mange, or serious infestations of parasites have been seen.