Svendsen (Weasels in Wild Mammals of North America, 1982) wrote:
“Overall, weasels are more of an asset than a liability. They eat quantities of rats and mice that otherwise would eat and damage additional crops and produce. This asset is partially counter-balanced by the fact that weasels occasionally kill larger, nontarget species – beneficial animals and game species. The killing of domestic poultry may come only after the rat population around the farmyard is diminished. In fact, rats may have destroyed more poultry than the weasel. In most cases, a farmer lives with weasels on the farm for years without realizing that they are even there, until they kill a chicken.”
Damage to Livestock and Pets
Occasionally weasels raid poultry houses at night and kill or injure domestic fowl. They feed on the warm blood of victims bitten in the head or neck. Rat predation on poultry usually differs in that portions of the body may be eaten and carcasses dragged into holes or concealed locations.