"Ermine are highly adaptable predators, easily invading small burrows to feed on voles, mice, and young rabbits. They also eat earthworms, frogs, and squirrels, climbing trees and swimming if necessary. Mother Ermine teach their young to hunt. Litters of 4 to 9 young are born in nests that are often located in rodent burrows. The newborns are blind and helpless, but in six weeks are almost adult-size. In the summer, the Ermine's coat is brown, but in the winter it is pure white except for the tip of the tail, which stays black. Ermine population density tends to fluctuate as rodent populations fluctuate."

Links:
Mammal Species of the World
Click here for The American Society of Mammalogists species account