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.