Cats that live in the wild or indoor pets allowed to roam outdoors kill from 1.4 billion to as many as 3.7 billion birds in the continental U.S. each year, says a new study that escalates a decades-old debate over the feline threat to native animals.

The estimates are much higher than the hundreds of millions of annual bird deaths previously attributed to cats. The study also says that from 6.9 billion to as many as 20.7 billion mammals — mainly mice, shrews, rabbits and voles — are killed by cats annually in the Lower 48. The report is scheduled to be published Tuesday in Nature Communications.

"I was stunned," said ornithologist Peter Marra of the Smithsonian's Conservation Biology Institute. He and Smithsonian colleague Scott Loss and U.S. Fish and Wildlife Service biologist Tom Will conducted the study.

It's part of a three-year Fish and Wildlife Service-funded effort to estimate the number of birds killed by predators, chemicals and in collisions with wind generators and windows. About a third of the 800 species of birds in the USA are endangered, threatened or in significant decline, according to the nonprofit American Bird Conservancy.

For years, bird lovers and cat lovers have clashed over whether outdoor cats, not native to the U.S., should be euthanized or allowed to roam free in managed programs that include neutering. City councils, animal shelters and state wildlife officials have long struggled with the balance.

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