Scientists from the Smithsonian Conservation Biology Institute and the Fish and Wildlife Service estimated that domestic cats in the United States — both the pet Fluffies that spend part of the day outdoors and the unnamed strays and ferals that never leave it — kill a median of 2.4 billion birds and 12.3 billion mammals a year.I actually know the folks who did this study, and they are extremely sharp. The methods they used were conservative. Admittedly, only 30% of the birds are killed by oudoor pet cats--the rest are by ferals. But that still adds up to nearly a billion birds.
Please keep your cats indoors, folks, and encourage your friends and neighbors to do the same.