A new study finds that cats kill billions of birds and tiny rodents every year. Livescience.com reported that cats kill more than any other mammal in the U.S. In the journal, "Nature Communications" the study revealed that cats kill between 1.4 billion and 3.7 billion birds a year and between 6.9 billion and 20.7 billion small mammals every year.

Livescience.com reports that the bird deaths may account for as much as 15 percent of the total bird population in the U.S. according to Pete Marra who is an ecologist with the Smithsonian Conservation Biology Institute.

"The magnitude of wildlife mortality caused by cats that we report here far exceeds all prior estimates," the paper said, according to the CBC.

"A lot of these cats may go outside and go to 10 different houses, but they go back to their house and cuddle up on Mr. Smith's lap at night," Marra said.

However, the biggest culprits aren't the free-ranging cats that have homes, but rather feral cats without a home. It's estimated that each of these cats kill between 23 and 36 birds a year and between 129 and 338 small mammals, Marra said. There are also between 30 million and 80 million homeless cats in the U.S.

"Cats without owners are blamed for most of the deaths. There are about 30 million to 80 million such feral cats in the U.S., each of which can kill upwards of 200 mammals a year alone, the study reported," said the CBC.

Marra said a way to reduce the killings is to keep cats indoors for those that own them.

"Listen, Tabby, we're going to have a heart-to-heart talk about how much time you spend outside," he said owners should say to their cats.

However Bruce Kornreich a vetrenarian at Cornell University's Feline Health Center said to Livescience.com,  "It may be in some cases that cats may also be keeping other species that may negatively impact bird and other small mammal populations in check."

The CBC reported that researchers also said that large numbers of birds and mammals are being killed around the world, not just in the U.S.