vailers f trailers for movies. we wrote that for him. but it turns out it is all true. we all know cats eat birds. what stunned scientists behind this week s new study when you do thorough sleuthing on cats it turns out they re four times more vicious than previously imagined. every year they kill 12.3 billion mammals, like chipmunks and 2.4 billion birds, a feeding frenzy that is even endangering some rare birds. why? because the number of stray cats has exploded. and it turns out felines are much deadlier than we knew. he is the cutest serial killer you will ever meet. reporter: amy watts thought she knew her cat, booker t. but when scientists put a camera on the kitty, it captured carnage. the new numbers took me by surprise too. i never thought my wife and i were harboring three cold-blooded killers in our own home. but it is a real problem. a human problem. experts say we are the reason the cat population has exploded and we all have to do a better job of keeping them inside.