When I checked my nest boxes this week, about half were still occupied — by mice. When I gently probed the mass of dried leaves with a stick, it was only a moment before I had a mouse running down my leg.
Deermice and white-footed mice commonly use nest boxes intended for cavity-nesting birds, and they’re welcome during the non-breeding season.
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.
In fact, the genus name, Peromyscus, is Greek for “mouse with boots.” And though I’ve never heard a deermouse sing in the wild, they do communicate vocally. (Listen at www.sankey.ws/peromyscus.wav.)
Deermice 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.
Deermice (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 in subterranean dens, under logs and rocks, as well as in tree cavities as high as 50 feet above ground.
One reason deermice 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. In the fall deermice 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 deermice do not hibernate, these food caches come in handy during the winter.
Cold days are the best time to find deermice in nest boxes. Often as many as eight mice huddle together in one box to conserve body heat.
The nest usually consists of a mass of chewed 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 deermice 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.
The pups grow rapidly and wean at three-and-a-half weeks. At eight weeks of age they are sexually mature. Adults can produce up to four litters per year, if they live that long.
The high reproductive rate is balanced by a short life span; most Peromyscus live less than a year in the wild.
I really don’t mind finding deermice in my nest boxes, especially now. I just make a note to remove the mouse nests in March to make room for the feathered cavity-nesters I prefer. Displaced mice simply seek shelter in another den nearby.
But why tolerate or even encourage deermice? My wife asks this question repeatedly each fall as we struggle to keep the house mouse-free.
Fortunately, deermice have one important redeeming quality. Almost every predator eats them. They form an essential link in the complex, interconnected food chains that make up virtually every terrestrial ecosystem.
Deermice 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 the ecological link in the food chain that connects plants to carnivores.
By promoting the winter survival of deermice, I help provide predators a dependable food supply. Every time I see or hear a screech owl, for example, I thank deermice. And if mice are abundant in the spring, perhaps predators will raid fewer of the nests in my bird boxes.