About half of my nest boxes are still occupied -- by mice. When I gently probe the mass of dried leaves with a stick, it's only a moment before a mouse jumps out and runs down my leg.

Deermice and white-footed mice commonly use nest boxes intended for cavity-nesting birds. 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 (www.sankey.ws/peromyscus.wav).

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 grass and weed seeds 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.

I really don't mind finding deermice in my nest boxes, especially now. I just make a note to remove them 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 represent an essential link in the complex, interconnected food chains that make up virtually every terrestrial ecosystem.

By promoting the winter survival of deermice, I help provide predators a dependable food supply. And if mice are abundant in the spring, perhaps predators will raid fewer of the nests in my bird boxes.

Looking for more from the Post-Gazette? Join PG+, our members-only web site. You'll get exclusive sports content, opinion, financial information, discounts from retailers and restaurants, and more. Our introduction to PG+ gives you all the details.