Cats Way Deadlier Than Thought, Report Suggests

More than 12.3 billion mammals and 2.4 billion birds estimated killed by U.S. felines.
2:13 | 01/30/13

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Transcript for Cats Way Deadlier Than Thought, Report Suggests
And finally, we have learned that the number of cats roaming outside in america tonight is exponentially larger than ever before. And not just stray cats, but pets, too. So, some scientists decided to spy on them. What did they learn? Abc's dan harris tells us our furry friends may have a double life. Reporter: In the age of youtube, they play patty-cake, get stuck in sleeves, and even, in this viral video I made with the aspca, fly. But do not be fooled by all this alleged innocence. Reality might be closer to this. Behind the adorable exterior, lurks lethal intent. The serial killer purring in your lap. You will never look at fluffy the same. Reporter: Ben johnson voices trailers for movies. We wrote that for him. But turns out, it's all true. We all know cats eat birds, but what stunned the scientists behind this week's new study is when you do some thorough sleuthing on cats, it turns out they are four times more vicious than previewly imagined. Every year, they kill 12.3 billion mammals like chipmoneyings 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 because, it turns out, felines are much deadlier than we knew. He's the cutest serial killer you will ever meet. Reporter: Amy watts thought she knew her cat, booker t., But when scientists put a little camera on the kitty, it captured carnage. The new numbers took me by surprise. I never thought my wife and i were harboring three cold blooded killers in our own home. Today, I went back to the aspca, where we adopted the cats. On a scale of 1 to 10, how worried should I be that one of my cats is going to kill me in my sleep? Not likely. Reporter: 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. Dan, you will never look at your cats the same. Reporter: Maybe not. Thanks, ben. Dan harris, abc news, new york.

This transcript has been automatically generated and may not be 100% accurate.

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