Deer mice are the most abundant mice in Washington. They occur at sea level on the ocean beaches and above timber-line, even breeding in the Arctic-alpine Life-zone. They occur on the islands in Puget Sound, the lowland marshes and deciduous thickets of western Washington, the great conifer forests, the alpine cirques and mountain parks, and the deserts of eastern Washington. Almost everywhere they are the commonest mammal encountered. They make their homes in city dwellings, under logs or in hollow trees in the forest, in the burrows of larger rodents and, if necessary, in burrows of their own construction.
Nests are usually cup-shaped masses of soft material, 4 to 8 inches in diameter with central cavities 2 to 3 inches in diameter. Materials utilized include dry grass, shredded bark, rope, rags, newspaper, moss, cattail fluff, fur, wool, and feathers. One nest discovered in the attic of an old building near Coulee City, Grant County, consisted of the mummified body of a wood rat (Neotoma cinerea). The fur of the arched body of the rat formed one side of the nest, while the fur of the far side of the body had been plucked to form the remainder. In the extensive marshes along Lake Washington, King County, deer mice occupy the ball-shaped nests of tule wrens (Telamtodytes palustris). These are constructed entirely of fragments of cattail leaves and cattail fluff and are placed in the cattail rushes about four feet above the wet ground of the marsh. Other deer mouse nests were found in an unused typewriter, in the pocket of an old coat, in a window frame, under a piece of tar paper on the ground, in a cavity in the ground under a board, between a ceiling and a loose bit of roofing paper, under rocks and logs, in hollows in logs, and in an unused nest of a wood rat. Where plant cover is dense, nests are placed on the surface of the ground, as under thick growths of tumbleweed (Salsola) in eastern Washington.
Deer mice are definitely nocturnal. Of thousands of specimens trapped only a few were taken in the daytime. Two of these were taken in a rock slide at Pass Creek Pass, Pend Oreille County, on the same day, and some unusual conditions may have incited them to travel in daylight. Deer mice are active throughout the winter and their lacy footprints are seen on the snow in below-freezing weather. They are mainly terrestial but one race (oreas) is at least partly arboreal for it is often taken in traps set in trees for flying squirrels. One was caught in a trap set 50 feet from the ground. All are good climbers, and oreas is perhaps the best. One was watched as it climbed the rough cedar shake wall of a cabin, ran upside-down across three feet of rough ceiling, and descended the opposite wall head first. They are equally agile in climbing rocks and cliffs.
The ordinary gait of the deer mouse is a steady run. When frightened it may leap a distance of one foot or more. When surprised in its nest it usually remains in the vicinity, hopping about in confusion, returning time after time to the exposed nest.
The food of deer mice consists principally of vegetable material such as seeds, nuts, berries, and the soft parts of fleshy plants. Mushrooms and lichens are eaten. Insect remains are present in small quantities in most stomachs examined. Flesh is also eaten and small mammals caught in traps are often eaten by deer mice. Cannibalism appears to be common only in captivity.
Fig. 107. Distribution of the deer mouse in Washington. A. Peromyscus maniculatus oreas. B. Peromyscus maniculatus austerus. C. Peromyscus maniculatus hollisteri. D. Peromyscus maniculatus gambelii. E. Peromyscus maniculatus artemisiae.
Deer mice are often heavily parasitized by tapeworms and roundworms. Virtually all of the deer mice taken on Jones Island in the San Juan Islands had livers so infected by the eggs of a nematode worm as to be swollen to several times their natural size, and had a yellow, crystalline appearance. The maggots of parasitic flies often occur in the anal and genital region of deer mice, effectively sterilizing some individuals.