Mice, voles and shrews make networks of tunnels under the snow that they use for shelter and to travel in the winter. Credit: Courtesy of Grace Bartlet

It is said “all roads lead to Rome,” but for mice, voles and shrews in my yard, all roads appear to lead to the bird feeder. This was made evident when a January thaw uncovered evidence of tunnels under the snow in my yard.

In front of me was a curved snow drift with over a dozen small holes dotting the semi-circle from end to end. Each hole was about 2 to 2 ½ inches in diameter, likely made more pronounced by the thaw. These portals all converged beneath the bird feeder, where blue jays and woodpeckers had ejected the tiny yellow seeds called millet in their quest for sunflower seeds.

Clearly, other small creatures were awake and active in the between place of the subnivean zone.

The word subnivean is Latin meaning “under snow” with “sub” meaning “under” and “nives” meaning snow. The subnivean zone is a place that comes to life between the bottom of the snowpack and the surface of the frozen ground. It’s just the right size for mice, voles and shrews. Even though it may be bitter cold and blustery above the snow, in this narrow zone the temperature remains stable throughout winter, varying only a degree or two above or below freezing.

But this subnivean zone doesn’t occur unless conditions are right. First, there needs to be at least a layer of 6 inches of snow. Next, one of two things must occur: Either spaces form when branches, matted grasses or leaf litter hold the snow up, creating hollows and chambers, or as the ground warms the snow, water vapor forms that rises through the snow. As the water vapor rises it leaves a narrow open space between the ground and the bottom of the snow providing air and space in which the animals can work.

Grasses, plant stems and tree trunks provide air shafts so fresh air enters this zone and carbon dioxide doesn’t build up.

Credit: Courtesy of Grace Bartlet

In this narrow open space mice, shrews and voles, who have no winter camouflage, can travel about and minimize their time out in the open on top of the snow. They can easily make tunnels and live relatively protected lives as they move around in search of food (seeds, bark, grasses, moss, etc.).

These tunnels are not foolproof enclosures. Larger creatures, like foxes, coyotes and owls all are active throughout the winter. With their keen hearing, they can detect activity below the snow and dive in head first or talon first to secure a meal. Slender ermines can even burrow into and follow the tunnels in pursuit of their prey.

That January day, the evidence of the mice, voles and shrews was limited to their converging tunnels zeroing in on the bird seed. As the snow recedes in another month or so, their network of winding tunnels zigzagging across the field will be visible. For now, it is enough to stare at this subnivean hub of life and ponder the amazing under-snow world, so nearby, yet so often undetected by human eyes and ears.

Grace Bartlett lives in Bangor and is a Maine Master Naturalist who volunteers for Bangor Land Trust, Orono Bog Boardwalk and Hirundo Wildlife Refuge.