SCOTT SHALAWAY Abundant food begets many mice

House cleaning has never been high on my list of favorite chores -- unless I'm cleaning bird houses. I enjoy making the rounds each fall to remove soiled nests, but as often as not I find nest boxes occupied.
By November, deer mice and white-footed mice have usually moved into nearly half of the boxes on the trail. These two species, both members of the genus Peromyscus, represent one of the most successful and widespread groups of rodents in North America. Members of the genus Peromyscus are easy to recognize, but specific identification is difficult. All have white feet, light bellies, large ears, brown bodies, large dark eyes, and long tails.
Deer mice and white-footed mice are especially difficult to distinguish. They even live in the same habitats -- old fields, forest edges, pastures, forests. The only way biologists can precisely identify these mice is to examine their skulls and teeth.
Deer mice (hereafter used to refer to both species) spend much of their time on or below the surface of the ground, but they climb trees easily. They build nests under logs, rocks, and roots, as well as in bird houses and tree cavities as high as 50 feet above ground.
One reason deer mice are so ubiquitous is that food is usually abundant. They eat seeds of many common grasses and weeds and a wide variety of berries, nuts, buds, and fungi. During the summer they eat everything from gypsy moth caterpillars, grasshoppers and crickets to an occasional egg or baby bird.
Store vast quantities
In the fall deer mice store vast quantities of seeds and nuts in cache sites as varied as hollow logs, tree cavities, nest boxes, and even old bird nests. I've often found an old robin nest filled with a mound of dried leaves. Inside, I usually found a generous supply of small seeds. Because deer mice do not hibernate, these food caches come in handy during the winter.
Cold winter days are the best time to find deer mice in nest boxes. Often as many as six mice huddle together in one box to conserve body heat. The nest usually consists of a mass of chewed dried leaves lined with fine grasses and fur. They keep their nests clean by using a separate nest chamber as the latrine.
In the spring these groups disperse, and pairs of deer mice set up housekeeping and breed. After a gestation period of 23 days, three to seven young are born. They are pink, naked, blind and helpless. They weigh one-and-a-half grams and measure about two inches at birth. The pups grow rapidly and wean at three-and-a-half weeks. At eight weeks of age they are sexually mature. Adults can produce three or four litters per year. If a female can survive for just eight months, she may give birth to as many as 28 young. A high reproductive rate is one way nature insures that small prey species remain abundant.
Remove the nests
I really don't mind finding deer mice in my nest boxes, especially in the fall. I just make a note to remove the mouse nests in March to make room for the feathered cavity-nesters I prefer. The displaced mice simply seek shelter in another den under a rock or hollow log.
But why encourage or even tolerate deer mice? Anyone who lives in the country knows how difficult it can be to keep a house mouse-free. Fortunately, deer mice have one important redeeming quality. They form an essential link in the complex food webs that characterize virtually every terrestrial ecosystem.
Deer mice eat primarily plant material. They, in turn, provide food to a tremendous variety of predators -- snakes, hawks, owls, weasels, raccoons, skunks, bobcats, coyotes, and foxes. Peromyscus are thus an essential ecological link in the food chain between plants and predators.
By promoting the winter survival of deer mice, I help provide predators with a dependable food supply. And if the population of mice is high in the spring, perhaps predators will raid fewer of the bird nests in my boxes.
sshalaway@aol.com