Chuck Raasch, USA TODAY

As many as 20.7 billion mammals%2C including mice and rabbits%2C also are killed by cats annually

Study is part of three-year Fish and Wildlife Service-funded effort

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 contiguous 48 states. 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 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.

"Our findings suggest that free-ranging cats cause substantially greater wildlife mortality than previously thought and are likely the single greatest source of anthropogenic mortality for U.S. birds and mammals," Marra and his co-authors conclude. "Scientifically sound conservation and policy intervention is needed to reduce this impact."

The study is critical of the Trap-Neuter-Return (TNR) policy advocated by Alley Cat Allies and other defenders of free-roaming cats. The goal of the policy is to gradually reduce outdoor cat populations while avoiding widespread euthanasia policies in animal shelters. An estimated 4 million cats are euthanized in shelters annually, according to Nathan Winograd, founder of the No-Kill Advocacy Center in Oakland.

The new study calls the Trap-Neuter-Return policy "potentially harmful to wildlife populations" because it leaves so many predators in the wild. The authors also say the policy is often put in place by cities and counties without "widespread public knowledge" and without studies on the impacts of large feral cat populations on the environment.

Cat defenders say that the new estimates won't change their belief that cats are scapegoats for bird habitat loss, chemicals used in fertilizers and insecticides, and collisions with man-made objects. "Human impact is the real threat" to birds, says Becky Robinson, president of Alley Cat Allies, a group that defends outdoor cats. She says the Trap-Neuter-Return policy is growing because people see it as a way to protect birds without killing cats.

"This is not Sophie's Choice; this is not the American people voting to kill one animal over another," she says.

George Fenwick, president of the American Bird Conservancy, says the issue is not cats vs. birds but "a runaway and invasive population of cats" that are killing too many birds

Fenwick says that the study gives his side powerful evidence to take to policymakers that Trap-Neuter-Return isn't working, and to push for more responsible cat-ownership policies across the country. He says too many people have been led to believe that cats can live outdoors without harm to themselves or the environment. The surprising numbers in this survey, he says, "will undo a lot of previously thought things."

Marra and his colleagues extrapolated findings from 21 studies in the U.S. and Europe to come up with an estimate of 30 million to 80 million "unowned" cats and 84 million "owned" cats in the U.S., their kill rates, and other factors leading to bird predation.

They defined "unowned" as farm cats living in barns, strays living outdoors that may be fed by humans, and feral cats that fend for themselves — all of which might live alone or in colonies. The study notes that Washington, D.C., alone has an estimated 300 outdoor cat colonies.