 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 predation in 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 . 
