They often kill more than they can immediately eat and cache excess kills. An ermine spends most of the daytime in a den beneath a stone wall, rock pile, log, fallen tree, or abandoned building. A den may have three or four tunnels leading to it. Breeding habits are similar to those of the long-tailed weasel. The natal nest is underground, lined with leaves, grasses, fur and feathers.
Newborns are blind, pink and weigh about half an ounce.
Young develop rapidly. The male may help the female care for them. Longevity is estimated at five or six years. An ermine's home range is thought to be about 30-40 acres, and 20 individuals have been found per square mile of good habitat. In winter and early spring, ermines travel long distances for food, often 2-3 miles per night.