On December 1, 1853, the paddle-wheel steamer Winfield Scott ran aground near Anacapa Island off the southern California coast. All 400 human passengers swam safely to shore. So did a legion of stowaway rats.

Meanwhile, 350 miles south, whalers and seal trappers periodically left goats on Guadalupe Island to establish a reliable source of meat. A few cats snuck off the ships and followed the goat herds onto land, too.

Like Anacapa and Guadalupe, most of the dozens of small islands off the Pacific coast from southern California down to Baja California, Mexico have their own story of how non-native mammals—rats, cats, goats, sheep, rabbits, or pigs—reached their shores. Wherever they landed, the introduced mammals bred unchecked and wreaked ecological havoc on their new homes. The invasive predators hit Scripps’s and Guadalupe Murrelets, which nest on some of the islands, particularly hard. Rats ate their eggs, goats chomped through bushes hiding their nests, and cats prowled for stray chicks.

“I’ve personally seen chicks that had been chewed on and died from rat predation,” says Anna Weinstein, director of Audubon California’s seabird and marine program. “It’s an ugly sight.”

A century after introduced mammals overran the islands, scientists finally led a proper census of the murrelets in the 1990s. It's not an easy task: The birds spend the majority of their lives at sea alone or in small groups, and they often nest in the inaccessible nooks and crannies of steep cliffs to avoid the introduced predators. Even seasoned birders grow excited at the prospect of observing one of the birds, which sometimes requires taking a small skiff out to the islands and climbing rocks at night.

What the scientists found worried them: They counted only a few thousand murrelets, and those that survived were surrounded by and defenseless against predators.

They became more alarmed when oil companies planned to drill offshore of nearby Long Beach and San Diego. So in 2002, the Pacific Seabird Group—a group of seabird researchers and conservationists—petitioned the U.S. Fish & Wildlife Service (USFWS) to consider listing the murrelets as threatened under the Endangered Species Act.​

But they didn’t wait to hear back from the agency before getting to work to protect the birds. Over the two last decades, an alliance of U.S. and Mexican environmental groups and government agencies—including the National Park Service, Audubon California, Island Conservation, and the Grupo de Ecología y Conservación de Islas—ramped up removal and restoration programs on the offshore islands. Anacapa Island rats were targeted with custom-made poison pellets sprinkled by helicopters, and conservationists replanted native vegetation. Other introduced mammals such as rabbits, pigs, and sheep were either shot from above or trapped by hand on both Guadalupe and San Benito islands, the only known breeding sites for the Guadalupe Murrelet.

Killing animals is never an easy decision, says David Mazurkiewicz, a biologist with Channel Islands National Park. But considering what’s at stake, it's sometimes a necessary evil. “We’re really talking about the extinction of species,” he says of the murrelets. On Guadalupe Island alone, several terrestrial landbirds—including the Guadalupe Bewick's Wren, Guadalupe Spotted Towhee, and Guadalupe Caracara—have already gone extinct from invasive predation and impact.

The conservationists’ hard work has paid off. On September 21, USFWS announced that the birds don’t qualify for protection under the Endangered Species Act. Not because their populations are expanding—indeed, only a few thousand breeding-age birds of each species remain. Rather, it's because "the stressors impacting the species have either been eliminated or reduced to the point" where there was no indication that the birds were at threat of extinction, the agency wrote. On the four islands that host 80 percent of the Scripps's Murrelet breeding population, no introduced mammals remain; on Guadalupe and San Benito islands, the only known breeding grounds for Guadalupe Murrelets, introduced non-predator mammals have been entirely removed. As a result, many native plants have returned and scientists expect the murrelets to recover.

“Watching the islands rebound like they have has been mind-blowing,” says biologist Robert McMorran with USFWS. “Seeing the land reclaiming itself is that immediate reward we all seek and don’t often get.”

Federico Méndez-Sánchez, development director with Mexico’s Grupo de Ecología y Conservación de Islas, recalls the satisfaction his colleagues expressed upon watching the “puffy little birds” running off Guadalupe Island cliffs into the sea. “It’s really amazing to see the murrelets thriving,” he says. “It’s been a fantastic collaboration.”

The work on Guadalupe Island was Mexico’s first big attempt at restoring an island's native wildlife. But the work isn’t done, Méndez-Sánchez warns: Feral cats still stalk native prey there. His team recently erected an 800-foot-long fence along its southern edge to keep the cats out and entice murrelets to breed on the main island, instead of its craggy islets. He estimates that it will take another $4 million to trap or kill the entire cat population, which at its peak reached 1,000 animals.

As murrelet monitoring continues, scientists are also keeping a close eye on other bird species affected by mammal invasions on islands throughout the Pacific—cormorants, herons, shearwaters, auklets, and especially Ashy Storm-Petrels, which have hovered on the edge of Endangered Species Act listing. To survive, they need a level playing field against the feisty introduced predators. “Seabirds are really responsive to island conservation,” says Audubon’s Weinstein. “They’re really tough. You give them a chance, and they come back.”