From Audubon Magazine

How to Catch a Rat

In 2019 a Mexican island that harbors 95 percent of the world’s Black-vented Shearwaters received an unwanted vermin visitor. To nab the interloper, Isla Natividad’s human inhabitants had to get creative.



Prologue: Charly Smells a Rat

Juan Carlos Ruiz Miranda had spent many early mornings chasing furry pests from his garden on Isla Natividad, a windy stretch of sand off Mexico’s Baja California Peninsula. If it wasn’t an invasive ground squirrel, known locally as the juancito, bothering his plants, it was a tiny cactus mouse—­Natividad’s only native terrestrial mammal. But one morning before dawn in spring 2019, as he returned home from a shift at the water treatment facility, Charly (as he’s known to Natividad’s 300-odd residents) spotted an unusual body skittering past his mallow bush. Its outline was that of an out-of-place critter, but since he saw barely a shadow, Charly dismissed it.

He next glimpsed the four-legged creature about a month later, in the wee hours of June 14. This time he shone a flashlight into his shrubbery to get a better look. The figure’s long tail, protruding ears, and fuzzy posterior were way too bulky to be those of a cactus mouse. When it darted back through the beam, Charly became certain: His tiny island harbored a big rat.

In most locales, a predawn rat sighting would cause no greater alarm than a shudder, since members of the genus Rattus have set up shop in ecosystems around the globe. But after nearly two decades of working on Natividad, and having grown up on a neighboring island, Charly knew this sensitive ecosystem had evolved without the presence of rats—or any mammalian predators.

He also knew that what Natividad lacks in mammals, it makes up for in birds: auklets, cormorants, pelicans, ospreys, and herons, to name just a few. More importantly, he knew this four-mile-long island hosts the planet’s largest colony of Black-vented ­Shearwaters—gray pelagic birds known for their shoreline flights, raspy cries, and nocturnal tendencies. A whopping 95 percent of the species hatch within a few miles of Charly’s yard, in labyrinthine underground burrows that honeycomb the island’s south end. For ages their hidden nests and crepuscular habits were the only defenses the seabirds required. 

Like many Natividad residents, Charly saw Pardela Mexicana as not just a bird bearing the name of his nation, but one connected to the soul of his island. To Natividad’s humans, this shearwater is their bird. Its name adorns a rainbow-hued mural of bird wings on the community center wall—a battle cry to protect the unique winged creatures of Charly’s home. And rats, he knew, might spell disaster for the beloved bird, so Charly ran to raise the alarm.



Act 1: Mobilize the Rat Force

Five hundred miles north and a few hours later, news of Charly’s rat sighting reached Mariam Latofski Robles in Ensenada. Latofski is development director of Grupo de Ecología y Conservación (GECI), a nonprofit dedicated to the conservation of Mexico’s islands. June is a busy month for GECI, which deploys field agents on both east and west coasts to monitor seabird nests and fortify colony habitats. Even though the office was swamped, Latofski shifted her focus to Natividad’s “biosecurity threat,” as people in her field label an intruder that can endanger native species. “It’s always a bit of a shock when a biosecurity alert is raised,” Latofski says. The damage invasive predators can cause—to say nothing of the immense efforts required to eradicate them—meant that her team needed to get moving.

Shearwaters and other wildlife that depend on an island’s fragile biome are among the animals most at risk from invasive species. Most evolved in isolation, free from terrestrial predators, until humans arrived on their islands with other creatures in tow. While cats, pigs, rabbits, and grazing mammals pose their own serious threats, rats perch at the top of this list, having reached more than 90 percent of the world’s islands. The rodents are responsible for at least 40 percent of bird and reptile extinctions worldwide.

The biology of a shearwater runs directly opposite that of Rattus rattus—alias the black rat or, in a nod to the vessel that spread them globally, the ship rat. While shearwaters produce a single egg annually, a female rat averages five pups per litter and has multiple litters a year. This fecundity has helped rats thrive virtually anywhere, spreading from southeast Asia across the globe. The Black-vented Shearwater, on the other hand, has a nesting range that almost entirely consists of a square mile along the tip of one small island. And the attributes the two species share—like nocturnal tendencies and a penchant for burrowing—only place the shearwater at further disadvantage, should a rat arrive on its sandy turf.

Since the 1990s GECI and other organizations have removed 60-plus introduced mammal populations from 39 of Mexico’s islands. Natividad’s goats—notorious chompers and stompers of crucial island flora—were removed in 1997; its dogs, known to harass and hunt seabirds as well as spread disease, were exiled four years later. Before a 2000 feline-eradication project, two dozen feral cats were killing around 45 Black-vented Shearwaters each month—per cat. These eradications, combined with outreach and targeted techniques to attract birds, have spurred a shearwater comeback. A 2019 paper reports the number of Black-vented Shearwaters nearly doubling within a decade to an estimated 115,000 breeding pairs, proving how crucial a predator-free island is to the birds’ survival. 

As Latofski weighed the risks of the predator Charly spotted, she considered the likelihood of predators, plural. “That’s how the literature says it happens,” she says. “If a person sees one rat, it usually indicates an established but small population.” Another potential plot twist: What if the rat Charly saw was solo but pregnant? Biologists estimate a single litter can multiply exponentially among itself, overtaking an island of Natividad’s size within two years.

So, though Charly’s was the sole rat sighting—and though some of his neighbors dismissed the alarm as one of his signature pranks—GECI sprang into action.



Act 2: Lay the Traps

On an island without native rodents, it might have been helpful to spread poison, but thanks to Natividad’s cactus mice, rodenticide was off the table. Within hours of Charly’s news flash, GECI had set every available trap around his cliffside house. These included live Tomahawk cage traps and Sherman metal box traps, plus small camera traps that would photograph any creature lured by their peanut butter bait.

After a few nights of those traps coming up rat-less, it became clear that more resources were in order, which meant more funding. Be it a rabbit infestation, a dozen feral cats, or a rat in a garden, effective and responsible predator eradication isn’t cheap. That February GECI had secured $15,000 from the Sonoran Joint Venture, a binational conservation collective, to proactively develop a plan for future biosecurity breaches. Four months later GECI was already in the middle of one, so they shifted much of that funding from crafting a “what-if” protocol to solving a living, breathing, mallow-bush-skittering problem.

Biosecurity officer María del Mar Garciadiego San Juan arrived on Natividad two weeks after Charly’s sighting with dozens and dozens of additional live traps, plus scented wax cakes that would show a rat’s distinct bite marks. The cakes yielded no conclusive evidence, and every night, the live traps filled with cactus mice eager to score a free meal. Once a Sherman trap springs, it’s out of commission until a human resets the trigger. The team had to work around the clock just to keep the traps functional. As the staff trudged from trap to trap around the island, Natividad’s residents occasionally cheered the team with an old GECI slogan, “Vivan las Aves!” (Long live the birds!)

“We baited them at 6 p.m. and checked them at 10 p.m., 12 a.m., 3 a.m., and then the next morning,” Garciadiego says. “It was exhausting.” She remembers a few cooler nights when she would hold each mouse in her hand for a few seconds to warm its body before releasing it. Her colleagues joked that soon she’d be knitting dozens of tiny mouse sweaters. But after two sleepless weeks, there was still no sign of the rat.



Act 3: Call In the K-9 Unit

Among the many experts GECI employs is Merlina, a rat-sniffing Belgian Malinois. Normally this superdog works at Ensenada’s port, detecting stowaway cats and rats on the Navy vessels headed to an outpost 218 miles southwest, on Guadalupe Island. Now the dog was summoned to Natividad to be its deus ex Merlina. But first the team had to be sure she wouldn’t misidentify the scent of cactus mice—an island species new to her highly trained nose—as that of Rattus rattus. With characteristic resourcefulness, GECI imported a few cactus mice from a nearby island for Merlina to smell. Once she proved their odor wouldn’t distract her from detecting the rat target, she and her handler made the 17-hour trip south in early July.

Merlina sniffed the perimeter of Charly’s house and all around the restaurant. She sniffed car engines and garbage sites. She sniffed the recycling center, with its rat-friendly cardboard piles. As she made her way from the clinic to the school to the basketball courts, “Merly” caused a sensation on Natividad, where pets are forbidden because the island is part of El Vizcaino Biosphere Reserve, Mexico’s largest wildlife refuge. Reports of rat sightings increased significantly with the prospect of a dog stopping by to investigate.

Though Merlina didn’t find any rats during her week on Natividad, she did mark the scent of Rattus rattus in the places where residents had spotted the creature, which vindicated Charly and helped GECI narrow its efforts. More importantly, on her sniffing beat Merlina never signaled that she smelled multiple rats—no nests or colonies. Thanks to her work, the team was more confident that Natividad had a solitary invader.



Act 4: Go to the Film

Though GECI’s camera traps and video surveillance were set at the supposed rat hotspots, all they documented in those first weeks were scores of mousy trap-crashers. One blurry shot from late June seemed like it might depict a rat. “It was, for sure, a bigger animal than the mice on the island,” says Garciadiego. She sent the image to peer biologists in the United States, Canada, and New Zealand, but none could identify the critter. “They were like, ‘This is the worst angle,’ ” Garciadiego says, laughing.

On the island, the tale of the rat was getting taller. How it got to the island remained a mystery. Could it have swum the four miles from the mainland or hid in a shipment of vegetables? Maybe it tunneled under the sea? Sightings were frequent, especially among the children. Garciadiego does a keen impression of Natividad’s youngest voices telling big-fish stories about the rodent’s size: “It’s like a monster walking all over town!”

Garciadiego and company carried this youthful momentum into GECI’s inaugural Festival de la Pardela Mexicana, a three-day celebration of conservation featuring talks, games, and interactive exhibits. Kids and adults played music, toured booths, and even showed off their shearwater smarts in a Family Feud–style game show.

After the festival the team collected the traps, shut down the cameras, and returned to HQ to regroup. Garciadiego spent an August morning scrolling through weeks-old surveillance footage of…mice, mice, and more mice. Occasionally a Burrowing Owl offered a choice photobomb. “I was going through each photo like, ‘Nope. Nope. Nope,’ ” she says. “And then I saw it!”

Bingo. Both camera traps and video clearly showed a Rattus rattus in the sand on the first night of the festival, not far from the very bush where Charly had discovered it. Additional footage was clear enough to catch its most distinguishing characteristic: a bend at the tip of its tail. Now they knew what the rat looked like, they knew where it hung out, they knew it was solo. And, thanks to some key angles, they knew it was male—phew.



Act 5: Add Essence of Muskrat

September brought a surge of activity to the island, as shifts of teammates worked intense two-week rotations before being relieved by a new pair. “Everyone would show up like, ‘It’s gonna be me who catches him!’ ” says Alejandra Fabila Blanco, a member of GECI’s Natividad Seabird Project Team. Then they’d leave a fortnight later with decidedly less cockiness. “But the rotation helps, because the new arrivals hear the experiences of the people who just finished working, and they come in with fresh energy, saying, ‘Let’s try this and this and this!’ That way, you always have new ideas.”

To avert the cactus mice (and allow the team to get more sleep), they lifted the traps onto platforms fashioned from cut PVC pipe. They made the bait more difficult for mice to reach by hanging it in the traps, crafting tiny handmade baskets from scrap wire. At one point, Latofski supposed that a solo rat might hunger not for food, but for companionship. So the team caught a few rats in Ensenada, swabbed their bodies and droppings, then shipped the stinky cotton balls to be rubbed on the latest trap innovation. To better tempt the rat’s palate, they tried myriad baits: long-lasting vanilla, reheated leftovers, essence of muskrat gonad. Fabila remembers her mother helpfully suggesting they try using cheese. “She has a point,” Fabila admits, “because the cheese they have on Natividad is pretty good.”

September turned to October, bringing the season Black-vented Shearwaters spend offshore, and the rat chasers continued on the quieted island without satisfaction. Now the invader was caught on several cameras a night, avoiding all the traps and, in the process, making an annoying stop-motion surveillance film of its defiance. “He was just going around, eating garbage, and passing by the traps like, ‘No thank you; have a good night,’ ” Garciadiego quips. Charly’s then 11-year-old son, Lalo, had taken to calling the rat Chapito—named after the famously elusive Mexican drug trafficker El Chapo, who avoided capture for decades and escaped from prison twice.

Lalo wasn’t the only resident thinking creatively. Residents implemented a few of the waste management ideas that they’d developed in summer brainstorming sessions, clearing any litter that might distract Chapito from the trap baits. Volunteer crews went on Saturday town cleanups, and homes began binning their organic kitchen waste (rather than scattering it for the gulls).

The weary GECI team kept brainstorming, too; Latofski told the team not to scrap any idea. One amazing trait of conservation biologists is their punk-rock ingenuity. When one method fails, you troubleshoot it. And if the equipment isn’t doing the job, you go full-on MacGyver. They were as resourceful and crafty as the rodent that eluded them, bricolaging traps from everyday items: a water-filled paint bucket buried in the ground with a narrow plastic pipe tunnel peeking out; a cone of fence wire with a fake exit; an enticing tightrope across a metal drum, rigged to give way should four furry feet deign to cross it.

But the months of fruitless Chapito pursuit weren’t great for morale. Latofski knew her team was getting frustrated. “They said, ‘We do this for a living; how can we not get one rat?’ I kept trying to instill confidence, because rats are very intelligent creatures. They’re adaptable—that’s why they’ve conquered the world!”