On a remote island in the South Atlantic, common house mice have become unrelenting killers, consuming millions of endangered baby birds a year, a new study confirms.

The massacre is taking place on Gough Island (map), a British territory almost smack-dab in between the tips of South America and South Africa. The only humans living on the island belong to a small team running a weather station.

Gough Island has long been recognized as an important seabird colony, since it hosts roughly ten million birds of more than 20 species. The island is also thought to be the only breeding ground for the Atlantic petrel—2 million breeding pairs produce 1.6 million chicks a year.

The new study, though, reveals that the petrel chicks are in constant danger from house mice, which have grown to supersize proportions since being introduced to the island 150 years ago.

"The sheer densities of the numbers of birds there—that's why Gough Island is so special," said study co-author Ross Wanless, of the University of Cape Town.

"But the mice seem to be chewing away through those"—causing the researchers to fear that the mice could eventually wipe out the Atlantic petrel.

Aside from Gough Island, a second breeding population of Atlantic petrels used to exist on Tristan da Cunha, another island in the South Atlantic. But scientists think that group has vanished in recent years due to predation by black rats.

The House Mouse Underground

House mice have no predators on Gough Island, and in summer their numbers reach "stupendous densities—300 mice per hectare," Wanless said. (A hectare is about 2.5 acres.)

That means there are about 1.9 million mice on an island that's just 25 square miles (65 square kilometers). What's more, the mice on Gough Island now grow 50 percent bigger than normal mice, reaching up to 10 inches (27 centimeters) long, including the tail.

As other food sources diminish in winter, the mice turn to the huge numbers of bird chicks.

For example, mice have been known to attack and eat chicks of the Tristan albatross, a ground-nesting bird—even though an albatross nestling weighs 300 times more than a mouse.

For the new study, Wanless and colleagues examined the mouse's effect on the Atlantic petrel. The species is listed as endangered by the International Union for Conservation of Nature, which means the bird is facing a very high risk of extinction in the wild.

Scientists studying the mice on Gough Island had seen signs that Atlantic petrels were also being consumed, but the species nests underground, and "it required a bit more study to get a handle on the burrow-nesting birds," Wanless said.

For four breeding seasons, Wanless and colleagues monitored 178 petrel nests, using infrared cameras to keep tabs on any invading mice and inspecting the nests every week.

Based on their results, the team estimates that, of the 1.6 million petrel chicks born each year, 1.25 million are eaten by mice.

"If this has been going on at this rate for 150 years, which is entirely possible, there could have been 30 million pairs of Atlantic petrels" before mice were introduced, Wanless said.

"Birds Have No Idea What's Coming"

The mouse's ability to overcome the larger petrels is "something people struggle to understand," Wanless added.

"Essentially, these are babies. If you put an extremely hungry rat in a room with a human baby, [the infant] wouldn't be able to defend itself," he said.

"It's the same with these chicks. The birds have no idea what's coming, because they've never been exposed to anything that will enter their burrow."

By introducing mice to Gough Island, "we don't know how we've affected the marine ecosystem," Wanless said.

Scientists do know that the petrels offer key benefits to the island.

For instance, there's "the manuring effect, from seabirds pooping in their burrows all the time. The whole island is incredibly enriched," Wanless said.

And by digging burrows, the petrels help ensure that the rich soil is turned over regularly. Remove the birds, and the island's ecosystem would suffer, he said.

In general, the results emphasize the notion that "mice cannot be ignored as a potential threat to island fauna, and island restoration and management plans should routinely include eradication of introduced mice," Wanless and colleagues write in their study.

The good news is that Wanless thinks there's an eradication program that would work, although it'd be expensive: Seed the island with poisoned bait.

"I've seen how unbelievably hungry these mice are," Wanless said. "I know without a shred of doubt that if you put food out at the right time of year, those mice would eat it."