Here’s an alarming but little-known figure—stray cats and pet cats allowed outdoors kill 3.6 million birds every day on average in the United States, for a total of at least 1.3 billion birds per year. That’s most likely a sizable chunk out of the U.S. land-bird population, which the Smithsonian Migratory Bird Center estimates is around 10-20 billion. While habitat loss and climate change pose long-term dangers to birds in this country, recent research shows that outdoor cats currently kill more of them than any other threat caused by humans.

It’s not just a problem in the U.S. A 2011 study found that domestic cats have directly contributed to extinctions of 22 bird species on islands around the world, and threatened dozens more. Researchers in the United Kingdom estimated that 55 million birds fall prey to domestic cats there each year; in Australia, threats to endangered species led government officials to announce plans for euthanizing 2 million feral cats.

Cat predation of wildlife, in other words, is a worldwide issue. But here’s something else that stretches across borders: People love cats.  In the U.S., there are about 84 million pet cats, and around 46 million of them are allowed to roam outside. An estimated 30-80 million more live as strays. That’s a lot of cats, and many spend their days doing what they’ve done since the first cats were domesticated more than 9,000 years ago: hunting small animals. Humans originally used domesticated cats as efficient predators, protecting stores of food from vermin. But there’s little need for working cats anymore; these days, most people just think of them as gentle companions and Internet memes. But their instincts haven’t caught up to our evolving needs—cats are still highly effective stealth hunters. And our having them around in such numbers means trouble for birds.

Some cat owners aren’t aware of the problem; some are, but feel that the companionship they receive from their pet outweighs their small contribution to a broader issue. But some cat lovers are also bird lovers. Two of them, a birdwatcher named Nancy Brennan and a bird biologist named Susan Willson, have developed what they believe is a solution.

Brennan, 57, spent much of her career in conservation and environmental planning. She  grew up in rural New England, where cats lived indoors and outdoors; she and her husband, who live in the Vermont woodlands near Green Mountain National Forest, took the same approach with their cat George. But hunting opportunities near their home are abundant, and for months after they moved in Brennan became increasingly frustrated as George dragged bird after bird into the house.

The breaking point, Brennan recalls, happened on the first spring-like day of 2008, when she heard “a ruckus” coming from just outside the house. It was George, struggling to pull a ruffed grouse, a gamebird the size of a small chicken, through the cat door. That morning, she vowed to either find a way to stop George’s hunting habits or bring him to the Humane Society.

Brennan already knew George couldn’t become an indoor cat, but her past attempts to keep him away from birds had failed. She had tried tying extra bells on his collar, but it seemed the cat moved too stealthily for the bells to have any sort of warning effect on his prey.

Then she recalled something she’d read about birds—they have excellent color vision. Birds have four color pigments in their eyes, compared to three in primates and just two in other mammals. While this adaptation helps birds find food and choose brightly colored mates, Brennan realized she might be able to put it to another use. She took up her sewing tools and gathered some multi-patterned fabric, piecing together something that resembled a ruffled Elizabethan collar with a bright color scheme. She fastened it as a cover over George’s usual collar and let him outside.

Sure enough, George returned home later that day without any birds—and none the next few days, either. As spring and summer passed without a single bird, she began to believe that she might be on to something that could work for other cat owners, too. She began tinkering with the prototype and created a website to sell the collar, which she named Birdbesafe. Over the next few years, she used customer feedback to zero in on which colors and patterns worked best.

The collars began to sell steadily, but they still remained scientifically unproven until 2013, when Willson, who studies tropical birds at St. Lawrence University stumbled upon the Birdbesafe website while looking for a way to rein in the hunting habits of her cat Gorilla. Soon after she brought him home, Gorilla began presenting Willson with dead birds, generally about two each week—a behavioral remnant, she believes, of his time as a stray, when he survived by catching and eating birds. “I’m a bird biologist. That was not a good thing, that was horrifying,” she said. Intrigued by the anecdotal evidence on the Birdbesafe site, she ordered a collar cover. Gorilla was beaten at last—he still caught voles, but he stopped bringing home birds altogether.

Intrigued by the collar’s success, Willson contacted Brennan and explained her idea for an experiment. She enlisted a group of cat owners near her home in Canton, New York, all of whom were dealing with bird-hunting pets of their own. She divided the cats into two groups, one that wore collars and one that didn’t; every two weeks, the Birdbesafe group and the control group switched places. Over the course of that fall, the cats brought home 3.4 times fewer birds while wearing Birdbesafe collars. The following spring, the collar covers made an even bigger difference—the cats killed 19 times as many birds while in the control group than while wearing Birdsbesafe.

“It was spectacular,” Willson said. She speculates that the difference was larger in the spring because birds are distracted from watching for predators at that time of the year, when high levels of hormones like testosterone cause them to focus on breeding behavior. The collar cover gave birds extra warning during the season when they’re least watchful.

Willson’s study was published earlier this year in the journal Global Ecology and Conservation. A few weeks after it came out, Australian researchers published a similar study in the journal Applied Animal Behaviour. This second paper found that Birdbesafe wasn’t just effective for birds—compared to control animals, cats wearing the collar killed 47 percent fewer animals with good color vision, a group that also includes reptiles.

For now, Birdsbesafe is available in scattered pet stores and bird-supply shops in 16 U.S. states and four other countries. Brennan says that since the scientific papers came out, sales have been greater than in all past years combined.

But some animal experts remain skeptical that the collar can be a large-scale solution to the problem of cat predation. “There’s some value to it,” said John Carroll, a biologist at the University of Nebraska who has studied the issue, “but it doesn’t get to the root of the problem” of the environmental damage caused by free-range cats. This goes beyond simply killing things—by competing with native predators for food, carrying diseases to other species, causing stress in birds and other prey animals, and mating with native wildcats, domestic cats can cause wide-ranging harm in fragile ecosystems. The Australian researchers that tested Birdsbesafe also concluded that while it helped save birds, it was not appropriate for protecting endangered mammals, which rely on smell and don’t pick out bright colors.

Brennan said she doesn’t see her collars as a pass for pet owners to let their animals live largely outdoors. Instead, she sees Birdsbesafe as an answer for people with cats that are unmanageable indoors. “This is another solution so we can keep chipping away at that problem,” she said.

Willson believes that Birdsbesafe collars could be used in feral-cat colonies as well. Feral cats kill more birds than owned cats do, she said, and their numbers are huge. Currently, Willson is preparing to test Birdsbesafe in France and at a handful of other sites around the world. New Zealand biologists just announced plans to test the collar covers as well.

In the meantime, on a much smaller scale, the collar has managed to solve at least one problem: Brennan’s cat “started sleeping in” instead of stalking wildlife.

“Some of them just retire,” Brennan said. “He had never missed a dawn hunting until he had been wearing my contraption for about a year. He was just like, ‘Oh, forget it.’”