Livestock depredations by black bears occur mostly in spring. A limitation of food sources in early spring and wild berry and nut crop failures during summer months are probably major contributing factors. Black bears can do extensive damage in some areas of the northwestern United States by stripping the bark from trees and feeding on the cambium. Though black bears will attack adult cattle and horses, they seem to prefer sheep, goats, calves, and pigs. They normally kill by biting the neck and shoulders, though they may break the neck or back of prey with blows from the paws. Evidence of a bear attack includes claw marks and is frequently being found on the neck, back, and shoulders of larger animals. Surplus killing of sheep and goats are common. Bears have been known to frighten livestock herds over cliffs, causing injuries and death to many animals; whether or not this is intentional is not known.