A 2013 study by Scott R Loss and others of the Smithsonian Conservation Biology Institute and the U S Fish and Wildlife Service suggested that free ranging domestic cats mostly unowned are the top human caused threat to wildlife in the United States killing an estimated 1 3 to 4 billion birds and 6 3 to 22 3 billion mammals annually Although quoted this statements seems ridiculously high Considering there is around 70 million cats in the US I wouldn t be surprised if those numbers were off by 3 magnitudes