“My friends call me the rat killer,” says Ralph Brabham, standing in his spacious backyard in Shaw on a sunny day in August. Brabham is a gentle man who hardly seems capable of splatting a spider, much less offing a rodent. But with a rat burrow on his neighbor’s side of the fence, he had no choice. “I put out a ton of traps,” he says of one battle. “I caught three rats in 30 minutes.”

Brabham had last spotted a rat in late July. The next time the unwelcome guests visited, they would meet a new weapon of rat destruction: Gypsy, a cat with a proclivity for pest control. Or so Brabham hoped.

Gypsy is one of more than 270 former strays enrolled in the Blue Collar Cats program run by the Humane Rescue Alliance. Two years ago, the organization created a third way for feral cats unfit for adoption because of their misanthropic tendencies. Previously, such cats were candidates for euthanasia. But with this program, available in the greater Washington area, the animals can remain semi-wild: They reside outdoors but under the care of a homeowner or business. In exchange for sustenance and shelter, they patrol alleyways, sidewalks, backyards and parks rife with rodents and other nuisances. Their uniform is a tipped ear — just a little snip at the top. “These are working cats,” says Lauren Lipsey, the alliance’s vice president of community programs. “They don’t ask for much because they don’t want to be around you anyway. It’s a mutually beneficial relationship.”

Of course, it’s the cats — not the humans — who decide whether they’ll fulfill their obligations or go rogue. “This one never left the patio,” says Lanier Heights resident Michelle Lee Chase, nodding toward Sundance, who was napping on Chase’s hallway rug one September afternoon. “But Butch has killed over 150 times.” Butch would be the more predatory half of the bonded pair. “All of our rodent problems went away.” (In defense of Sundance’s early retirement, last winter he was attacked by a raccoon squatting in his shelter and lost a leg in the fight.)

Blue Collar Cats has a waiting list of about 50 homeowners and businesses: It seems that not all feral felines are cut out for the exterminator trade. Plus, the organization tries to find the best environment for each animal. It looks, for example, for properties with some shade and quiet green space that aren’t too close to high-traffic areas or on a block with a lot of other Blue Collar Cats. A townhouse with a leafy backyard in Shaw is a match; a shopping center with a giant parking lot is not.

To participate, cat keepers pay a $50 placement fee and supply food, water, treats and toys year-round, plus a warm habitat during the colder months. They must also commit to caring for the animal for its lifetime. If they move, they can pack up the cat or bequeath it to the next resident, as long as the new tenants accept responsibility.

Brookland residents David Forster and Rebecca Caldwell upheld their end of the agreement, but three of their four cats ignored the pact. As part of the acclimation process, all cats spend several weeks in a large enclosure on the grounds of the site to establish a home base. During the transfer from the transport crate to the pen, Barney dashed off, never to return. Tuna, the couple believes, was later catnapped, while Socks, a replacement for Barney, apparently ran off with a brawny stray. Blue Ivy, who arrived in March, stayed and has since started dabbling in domestication.

“Oh, look at him,” Caldwell coos, as the tuxedo cat rubs against her leg in the kitchen. “Yes, I’m talking about you. I don’t know what he does during the day. He could probably transition into a house cat. Don’t! No biting! Time for you to go outside.”

Caldwell opens the door and Blue saunters out to the back porch, where he picks at his bowl of food. As the sky begins to darken, he sets off to work. “He’s hunting leaves,” says Forster as Blue pounces on a pile of debris on the lawn. After the pair retreat indoors, Blue ventures onto the sidewalk and crosses the street, disappearing in the shadows.

Forster and Caldwell haven’t seen any evidence of rats, dead or alive. That’s not surprising. “The cats are a rodent deterrent,” explains Lipsey. “You won’t find an accumulation of rodent bodies.” Across town, however, the Chases have mounds of proof. Butch brings all his conquests home, depositing the bodies on the back deck like FedEx packages. The 2-year-old gray targets a wide range of critters, including mice, bugs and birds. (Asked about the threat to birds, Lipsey says, “Ultimately, we do endeavor to decrease the number of cats living outside.”)

During Gypsy’s confinement, Brabham seemed to be the only prey in her crosshairs. In an attempt to bond with her, he tried to feed her a treat. She “hissed like a demon” and scratched his hand. “I feel bad for the rats that she encounters,” he says.

Before her release in mid-September, Brabham fed her a feast of baby-food chicken, kibble, hot dogs and Temptations treats, basically Skittles for felines. He carefully opened the enclosure and scurried to the back fence to give her a wide berth. Gypsy didn’t budge. Brabham went into the house, and she stared at him through the glass door with angry yellow eyes. As the minutes clicked by with no movement, he left Gypsy to decide how she wanted to live her life — as a free cat or a confined cat.

Later that afternoon, Brabham returned home to find an empty pen. A neighbor reported a Gypsy sighting on a nearby roof. The following day, Brabham caught a glimpse of Gypsy eating her breakfast. He also noticed that she had refurnished her home office, positioning her toys around the backyard to her liking.

Andrea Sachs is a Wasghington Post staff writer.