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Last year, one of the world’s most aggressive island restoration projects was launched to poison all the invasive rats on Alaska’s Rat Island, located in the western part of the Aleutian islands. But the extermination project may have taken an unexpected toll: a recent survey of the island recovered the corpses of 41 bald eagles and 186 glaucous-winged gulls – raising the possibility that the birds died after consuming poisoned rats.
“When you go to an island after a winter, it's not surprising to find bird carcasses,” says U.S. Fish and Wildlife spokesman Bruce Woods, “but not these numbers.” There were only four breeding pairs of the federally protected bald eagles residing on the island last year, but the population in the Aleutians numbers 2,500.
Norway rats—the global pest of subways and sewers—first arrived on the island known to the Aleut people as Hawadax in 1780. A rodent-infested Japanese sailing ship ran aground and rats spilled onto its rocky shores, devastating populations of ground-nesting seabirds by feeding on their eggs, chicks, and even adults. When Captain Fyodor Petrovich Litke visited the island in 1827, he named it Kryssei, which means rat in Russian. The rats have kept bird numbers low ever since.
In 2006, the U.S. Fish and Wildlife Service, the Nature Conservancy and Island Conservation began testing out rat eradication strategies in small areas in other parts of the Aleutians using cakes of grain laced with a rodent-killing blood thinner called brodifacoum. During the trial, 88 percent of rodents perished in their burrows where they would not be exposed to scavengers like eagles or gulls, according to an environmental assessment of the project. Scientists concluded that “some bald eagles may be exposed to brodifacoum residues but are unlikely to die.”
On April 15, 2008, the Rat Island Restoration Project got the formal go-ahead, and in September a helicopter hovered over the 10 square mile island dropping the poisoned grain in a grid pattern. “The idea is to put every rat in jeopardy, to spread the bait in sufficient density so that every rat will consume a lethal dose,” the Nature Conservancy’s Steve MacLean told American Way magazine at the time.
Immediately after the poisoning, wildlife workers scoured the ground and found only a dozen rodent carcasses. “There were not a lot of piles of rodent bones or rodent fur that would be expected if a lot of rats had been consumed on the surface,” Woods says. If scavenging indeed was minimal, then something else killed the eagles and gulls.
In any case, the recent tragedy represents a blow to a powerful restoration strategy that has already pitted conservationists against landowners and animal rights activists in New Zealand, California, and Hawaii. The corpses were sent to the National Wildlife Health Center’s laboratory in Madison, Wisc., and a determination on the cause of death should be available in late June. At least 75 percent of the carcasses were juveniles.
The good news is that the survey team reports that Aleutian cackling geese, ptarmigan, peregrine falcons and black oystercatchers are all nesting on the island -- and rats appear to be absent. “If the rats are truly gone,” Woods says, “then the long-term benefits to the ecoystem will outweigh the loss of these birds.”
Image of bald eagle courtesy Alex Layzell via Flickr.