A small mouse with a huge range (the Northeast, Midwest and the West from Alaska to Mexico), the deer mouse occurs throughout Pennsylvania. It is 6 - 8.5 inches long, including the tail, which is three to four inches. A deer mouse weighs 0.4 - 1 ounce. For the first month of its life, an individual is colored gray; then it molts into its brownish-gray adult pelt. In both juveniles and adults, the undersurfaces are pure white. The deer mouse has large dark eyes well adapted to night vision. Deer mice inhabit nearly every type of land habitat in Pennsylvania: farm fields, fencerows, grassy berms of roads, brushland and deep woods, both dry and damp, pine and hardwood. (Some taxonomists recognize two forms of Peromyscus maniculatus, the “woodland deer mouse” and “prairie deer mouse.”) Deer mice eat seeds of many plants, cultivated grains, soybeans, corn, berries, buds, nuts and mushrooms. They consume beetles, grasshoppers, crickets and caterpillars (including those of the gypsy moth); other invertebrates such as earthworms, centipedes, slugs and spiders; and carrion. Deer mice have sharp hearing and good eyesight. They locate most of their food by smell. They can swim if necessary and run at nearly five miles per hour for short distances. The tail, covered with fur, acts as a tactile organ and a balancing aid; when climbing, a deer mouse wraps its tail around twigs or branches to gain steadiness. The species weaves ball-shaped nests, 6 - 8 inches in diameter, out of leaves, grasses and other plants, lined with fur, feathers and shredded plant matter. Deer mice nest in hollow logs, stumps, fence posts, beneath rocks, in root channels underground and, rather frequently, in abandoned squirrels’ and birds’ nests in trees up to 50 feet above ground.
Deer mice rest in their nests during the day, and there they rear their young. Nests at ground level may have a nearby burrow with a latrine area for waste and a chamber for storing food. In winter, if snow covers the ground, deer mice spend most of their time beneath the white blanket, where the temperature is warmer than in the open air. They eat stored food. In extreme cold, deer mice cut down on their activity, sometimes sleeping for several days, perhaps huddled in a communal nest with two to four other mice (some of which may be white-footed mice, a different, although closely related, species). It’s common for people to find deer mice using bird boxes in winter. Deer mice breed from March to October. Females raise 3 - 4 litters per year, each with 3 - 7 young. In one year, a female can produce nearly 30 young, although few survive long enough to do so. Young mice, called pups, utter high-pitched squeaking sounds. Males do not help females raise the litters. Deer mice are preyed upon by foxes, cats, short-tailed shrews, mink, weasels, hawks, owls and snakes. Home ranges vary from 0.05 - 2.5 acres, with three to 36 mice per acre of habitat. Like most other small mammals, deer mice are very abundant in some years and rather scarce in others.