Cats hunt small prey, primarily birds and rodents, and are often used as a form of pest control. Domestic cats are a major predator of wildlife in the United States killing an estimated 1.4–3.7 billion birds and 6.9–20.7 billion mammals annually. The bulk of the predation the United States is done by 80 million feral and stray cats. Effective measures to reduce this population are elusive, meeting opposition from cat enthusiasts. In the case of free ranging pets, equipping cats with bells and not letting them out at night will reduce wildlife predation. Feral cats and house cats that are free-fed tend to consume many small meals in a single day, although the frequency and size of meals varies between individuals. Cats use two hunting strategies, either stalking prey actively, or waiting in ambush until an animal comes close enough to be captured. Although it is not certain, the type of strategy used may depend on the prey species in the area, with for example, cats waiting in ambush outside burrows, but tending to actively stalk birds.