Weasels are wholly carnivorous. Though larger and more powerful than the Ermine, the Long-tailed Weasel feeds primarily on rodents; also chases squirrels, chipmunks; may take rabbits and shrews. It also hunts birds, especially ground-nesters, and can cause considerable damage in a chicken coop. It benefits man, however, by killing rats and mice in fields and barns. Weasels attack prey several times their size, climb 15-20 feet up a tree after a squirrel, and occasionally go on killing sprees. Their killing instinct is triggered by the smell of blood; even an injured sibling will be killed and eaten.
Like the Ermine, this weasel changes color twice a year, except in the southern part of its range. Molting occurs during a period of about four weeks, and the gradual nature of the process explains the part-white, part-brown individuals that are sometimes encountered. Molts are triggered mainly by changes in day length, though temeperature also plays a role. The white hairs of a weasel's winter coat first appear near the abdomen, then move up its flanks, and spread to its legs and face, allowing the animal to blend with fallen leaves covered with snow patches. Mating is usually in midsummers. Four to nine young are born blind, naked, in early May in abandoned dens of other small mammals. They disperse at 7-8 weeks, when males are already larger than their mother. Long-tailed weasels are widespread and fairly common throughout their range.