Saving a remote island’s birds—by getting rid of its mice

In his many years of documenting wildlife, Thomas Peschak has seen some bloodshed. When he first saw footage of what’s happening on Marion Island, however, the National Geographic photographer was in for something new. Amid this stormy jewel of biodiversity halfway between South Africa and Antarctica, he photographed the gruesome aftermath of mice creeping into albatross and petrel nests in the night and biting the skin off living seabird chicks.

“The sheer carnage that these mice create on albatross chicks is just mind-blowing,” Peschak says. “I have a tough stomach, but that’s pretty rough.”

Right now, only a small percentage of the island’s albatross population is dying from the attacks. But ornithologists anticipate the problem will get worse, which is why they’re launching one of the most ambitious mouse eradication projects on at island to date.

In 2020, BirdLife South Africa, a non-profit conservation organization, and the South African government will team up to drop poisoned bait via helicopter over every square meter of the island in an attempt to kill off the rodents. If successful, this will be the largest island cleared of a damaging invasive mouse population. If it fails, the rodents will continue cutting down not just the birds, but the whole island’s ecosystem.

The problem of mice

Mice were likely introduced to the island on accident by seal hunters some 200 years ago, but Marion Island researchers only started noticing scalped birds around 2009. After infrared camera footage revealed that mice were responsible for the attacks, biologists worked backward to figure out why particular birds were being victimized. These include three species of endangered and vulnerable albatross—grey-headed, sooty, and wandering—as well as near-threatened grey and vulnerable white-chinned petrels.

The researchers found that climate change is creating warmer winters that kill off fewer mice. As a result, the island’s population has outgrown its supplies of the mice’s normal food sources, including weevils, moths, and seeds, explains Otto Whitehead, an ornithologist who earned his Ph.D. at the University of Cape Town by working on the island.

Having already put a dent in insect populations on the island, the mice have had to find new food sources. And of all the prey for the mice to pursue, albatross and some petrel species appear to be the easiest targets. Albatross nests, perpetually warmed by the birds’ body heat, entice mice to burrow into the insulated ground below them, Whitehead says. Petrels’ underground homes are even better; mice simply move in while the birds are still there.

This gives the mice easy access to the seabird chicks. Albatross chicks, for example, are tended by their parents for the first two months after hatching, but then sit unguarded in their nests for about seven months as parents travel long distances to find food. The parents may only swing by once a week for an hour or two to drop off food for their young, so they can’t offer much protection.

It might take a couple nights of what the ornithologists call “nibbling” (a term that Peschak says is “the understatement of the year”) to kill a bird. Still, mouse predation accounted for about 10 percent of albatross chick deaths in 2015. Having never dealt with these attacks in the past, the birds have no defenses. Albatross chicks are kept awake all night by the biting, and then spend their days exhausted, attempting to recover from their injuries.

For some petrel species, who are more likely to remain near their nests, the stunned parents simply watch the attacks happen. “It’s like something out of a zombie apocalypse,” Whitehead says.

A solution dropped from the sky

The birds haven’t learned to fend off the mice themselves, so conservationists have decided to do it for them. Ross Wanless, an ornithologist with BirdLife South Africa, is one of the researchers orchestrating the 2020 poison drop meant to eliminate Marion Island’s mice. Though researchers have killed these rodents on other islands, Marion will be the largest patch of land they’ve attempted to clear of mice, he says.

Helicopter pilots will fly in a grid to drop one piece of poison per square meter of the Detroit-sized island. Since the rodents don’t travel far for food, this method should theoretically hit them all. But missing bait in even one 20-meter-by-20-meter patch could allow a few mice to survive and reproduce, which would ruin the entire mission.

If that doesn’t sound difficult enough, the pilots will also have to contend with extreme weather. Marion Island sits in a stretch of ocean notorious for its brutal winds. Even with extra days to accommodate impossible flying conditions, the pilots will have to be incredibly skilled to pull off this mission, Whitehead says. “It’s one of those insane locations that tries everything in its power to kill you on a daily basis,” Peschak says.

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The poison will be incorporated into grain, to attract the mice. After eating it, they will die. The remaining poison will degrade and wash into the ocean at undetectable levels, says Wanless.

Albatross are fish-eaters and so are unlikely to pick at the bait or the poisoned mice. But scavenger species like the lesser sheathbill, which will be overwintering on the island during the drop, just might. To protect them, another team of researchers will put the birds into temporary captivity. The sheathbills will be held and fed until the conservationists are sure the island is clear of fatal temptations.

All this precaution isn’t cheap. “Every day is tens of thousands of dollars of fuel and time,” Wanless says. “The planning has to be meticulous.”

An ecological trade-off

Despite the high stakes, conservationists think the eradication attempt is worth it. Dan Simberloff, an ecologist at the University of Tennessee, Knoxville who isn’t involved with the project, acknowledges that there’s some risk in members of other species being hurt. “Is it absolutely certain nothing awful will happen? No, but the probability is low,” he says. “And meanwhile, the mice are a disaster.”

Wanless feels similarly. He’s confident that the eradication will be successful, and even if a few birds are lost along the way, the resulting pristine ecosystem will be worth it, he says. Besides, the albatross population is already struggling from the ill effects of commercial fishing around the world; the birds often get caught in the hooks on ships’ long lines. “They’re getting clobbered at sea, and then they try and breed, and mice eat the chicks,” he says. “They don’t need that additional stress.”

After the drop, the researchers will have to wait two years to know whether their strategy worked. At that point they’ll be able to determine if seabird scalping has ceased and if insect populations have rebounded. Until such evidence appears, all the scientists can do is wait.

Though Peschak is hopeful, he says the final result will decide much more than just the fate of one island. “If a place as wild and remote... as Marion Island can’t be saved, what hope is there for the rest of the planet?”