First, because cats are protected by their owners from disease, competition and predation, they are immune to the factors that control wild predator numbers. Second, while populations of wild predators rise and fall with the availability of food, domestic cats survive on supplemental food provided by their keepers. Third, because cats aren't territorial their numbers can reach far greater densities than native predators. In some parts of rural Wisconsin, as many as 114 free-ranging cats have been counted in a square mile. They often hunt in packs or colonies of 20 or more cats, and a colony of 81 was once recorded.
Evidence suggests that even when fed regularly, a cat's basic instinct to hunt remains strong.
Studies show that up to 90 percent of a free- ranging cat's diet consists of wild animals. While many of the mammals killed by cats are mice, rats and other pests, cats also take a heavy toll on songbirds and mammals whose populations have been stressed by other factors. In Wisconsin alone, scientists estimate that cats kill 39 million birds a year and the number could be closer to 200 million. Experts encourage cat-owners to do their part to control the population by neutering females and keeping cats confined. Well-fed rural cats tend to stay close to farm buildings where they control rodents but don't disturb native wildlife.