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You've seen them out there — well-fed cats, sometimes with collars on, stalking the streets like they own them or collapsing on a warm sidewalk to loll in the sun.

Cat lovers find them charming. Wildlife conservationists and bird lovers see furry killers and blame them for a decline in the bird population and the deaths of untold numbers of voles, chipmunks and other small animals.

How you feel about outdoor cats may also depend on where you are in the world. In the United States, about 81% of domestic cats are kept inside, according to a 2021 demographic study of pet cats.

But elsewhere, it's far more common to let them roam. In Denmark, only 17% of cats are strictly indoor pets, according to the study. In Turkey, feral cats walk freely in and out of cafes, restaurants and markets.

In Britain, where 74% of cat owners let their felines roam outside, many cat charities advise pet owners on how to keep cats safe outdoors. The idea might be shocking to their American counterparts, which often refuse to adopt cats to people who want to keep their pets outside.

"We've always done it that way," said Nicky Trevorrow, a cat behaviorist at Cats Protection in Britain, which encourages owners to bring cats in at night and feed them high-quality diets to deter predatory behavior.

"As a behaviorist," Trevorrow said, "I would have to say very much that I'm in the camp of giving cats space to breathe and be outside."

But should cats have this much freedom?

During much of the 20th century, most cats stayed outside, said David Grimm, author of "Citizen Canine: Our Evolving Relationship With Cats and Dogs" and a deputy news editor at Science.

The invention of kitty litter in 1947 made indoor cats more acceptable. But it wasn't until around the 1980s that more Americans began bringing their cats indoors, as conservationists warned of declining bird populations and veterinarians cautioned that an outdoor cat was more prone to diseases, parasites and infections, and could be vulnerable to attacks from larger predators like coyotes and hawks, or speeding cars.

But many owners have also felt conflicted about keeping a curious, restless creature inside, said Grimm, who has trained his cats to walk on leashes when they're outside.

Keeping them inside "didn't feel right," he said. "Just like I wouldn't keep my kids inside all day. We can only take so much animal out of them."

Wildlife specialists beg to differ. Where cats have been introduced, they have decimated native creatures, according to a 2011 study by biologists.

"I feel pretty strongly that it is a pretty devastating invasive species," said Jason Luscier, an associate professor of biology at LeMoyne College in Syracuse, N.Y. He helped develop an app, called "Cat Tracker," to get a more accurate reading on the number of outdoor cats around the world.

Luscier said colonies of feral cats, which multiply easily and can overwhelm an ecosystem, are the bigger threat to birds and other wildlife, not outdoor domestic pets that come in at night and are fed regular meals.

The best solution, he thinks, is to adopt feral cats, have them spayed or neutered and domesticate them, a policy pushed by the Wildlife Society.

Trevorrow, however, said there are larger threats facing birds, such as loss of habitat and the commercial use of pesticides that kill insects, the natural prey of birds.

"I just feel like cats are used as a scapegoat," Trevorrow said.

The Royal Society for the Protection of Birds in Britain said the decline in bird populations has been caused primarily by human-made problems such as climate change, pollution and agricultural management.

While there is evidence that cats may kill up to 27 million birds a year in Britain, "there is also evidence that cats tend to take weak or sickly garden birds," said Anna Feeney, a spokeswoman for the organization. "Cats are unlikely to have a major impact on populations," she said in an e-mail.

Trevorrow has written guides for cat owners who want to keep their pets outside and maintain a garden that will attract birds and other pollinators.

"There is a way to have both without carnage," Trevorrow said.

Still, the best way to keep your cat — and wildlife — safe is to put it on a leash, keep it in a fenced in-area, or build a "catio" that will allow it to play outside without being exposed to the elements, said Dr. José Arce, a veterinarian and president of the American Veterinary Medical Association.

Susan C. Beachy contributed research.