A new study, published January 29 in Nature Communications, estimates that cats are responsible for killing billions of birds and mammals in the continental U.S. every year. The estimate: 1.4 billion to 3.7 billion bird victims and 6.9 billion to 20.7 billion mammals. Peter Marra, the senior author of the study, called the results "stunning." (Watch: A house cat's point of view.)

"For the last 20, 30, 40 years," he said, "the number that has been batted around as a max was about 500 million."

And there are a lot of potential feline killers. Over 80 million pet cats reside in American homes and as many as 80 million more stray and feral cats survive outside.

The authors found that the