*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.