As I write this, it is -25C at 4 p.m. and we only have a few centimetres of snow on the ground. This may not be good enough for the winter survival of small mammals who live in the subnivean zone, i.e., under the snow. The deeper the snow is, the better.
The subnivean zone is the area between the surface of the ground and the bottom of the snowpack. This is where mice, voles and shrews go for protection against the cold of winter and from predators. Food is readily available from the plants, seeds, bark, and insects on the ground.
The plants are already frozen by the time the snow arrives and their rigid frozen state can hold the snow up off the ground and help to create little cavities where these small animals can hang out. Eventually the weight of the snow will weigh the plants down, but the heat from the ground will counter it and create conditions favouring gaps between the ground and snow. In addition, there will be periodic “breathing” holes leading up into the air above the snow.
It takes only six inches of snow to create a protective cover above and room below for mice, voles, and shrews, but an additional two inches will help to maintain an ambient temperature around freezing.
It is in the gap that the animals create runways, food caches, sleeping areas, latrines and so on. Tunnels often start at the base of a tree or near a rock. These darker surfaces absorb heat from the daylight sun, and this is reflected into the tunnels for extra warmth.
The mice and voles often build nests with dead grasses into which they retreat in family groups to keep warm.
While they may be protected from the severity of the weather, mice and voles are not completely safe from predators. Owls such as the short-eared owl can hear the mice running around under the snow and plunge into the snow with their balled-up feet first to capture their prey. Once the snow has a hard crust the short-eared owls find it too difficult to hunt this way and they leave our valley.
Foxes and coyotes use smell to detect their prey. Foxes are well known for their acrobatic, nose- and front feet first-plunge into the snow and rarely miss their target. Weasels (ermine) can gain access to the runways by slithering down the air holes and are such successful hunters they leave little behind.
The runways and nests can easily be seen in spring just as the snow melts.