Hordes of giant mice are devouring endangered seabird chicks on a remote South Atlantic island and may be pushing some of the birds to extinction, scientists report.
The carnage has harmed the breeding success of endangered Tristan albatrosses and threatened Atlantic petrels on Gough Island, a British territory a thousand miles (1,600 kilometers) off the coast of South Africa.
The birds' sole breeding ground is home to 22 bird species—10 million birds in total—and is considered the world's most important seabird colony.
Common house mice were introduced to the island more than a century ago. Now three times larger than normal mice, the invasive rodents likely number more than a million.
Video cameras revealed one pack of ten mice feeding on a Tristan albatross chick's wounds as it nested on the ground. Footage also showed mice devouring Atlantic petrel and great shearwater chicks.
The birds did not fight off their attackers, even as some mice fed inside the body cavity of one albatross chick.
Researchers say the footage provides the first hard evidence that mice previously thought harmless to seabirds are willing to attack prey more than 300 times their weight.
"Like a House Cat Attacking a Hippo"
Nearly three feet (one meter) tall, a Tristan albatross chick can weigh up to 22 pounds (10 kilograms), or about as much as a turkey. Gough Island mice weigh just 1.2 ounces (35 grams) on average.
Geoff Hilton, a U.K.-based biologist with the Royal Society for the Protection of Birds and study co-author, has compared the mismatch to a house cat attacking a hippopotamus.
Hilton and his colleagues say that while the seabird chicks can defend themselves from other birds, the chicks don't appear to be programmed to fend off novel predators like mice.
Ross Wanless, a Ph.D. candidate at the University of Cape Town, led the study, published in the online edition of the journal Biology Letters.
"The mice on Gough are amongst the biggest wild house mice in the world. In winter, they eat through the 'island larder' until they start to run out of seeds and invertebrates," he said.
"At that point they increasingly start to eat seabirds, as well as other mice. A Gough winter is not a nice place to be a seabird or a mouse."
Wanless says Gough Island mice are depressing bird populations and will cause some species, such as the Tristan albatross, to go extinct if left unchecked.
The researchers believe mice become predatory when other introduced animal pests, such as rats and cats, are eliminated, freeing the mice from competition and predation.
While small in area, ocean islands sustain the world's highest diversity of bird species, the researchers note, adding that over the past four centuries, 90 percent of global bird extinctions have involved island species.
Many such extinctions have been caused or aggravated by rats, cats, and other introduced animal pests.
The researchers say that in light of their new evidence, island restoration programs should also target mice.