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Laurie Wein from Parks Canada watches as a helicopter carries rat poison for an air drop at Murchison and Faraday Islands in Gwaii Haanas National Park Reserve.

The two helicopters came in low, 50 metres off the ground, and began to make slow, merciless passes as they spread 45 tonnes of poison over the island.

This is how the war on rats began in Alaska, with a military-style assault that would hold valuable lessons for a project now underway in Haida Gwaii, an archipelago of 150 islands off British Columbia's northwest coast.

Unlike most of Canada, parts of Haida Gwaii escaped glaciation and the isolated region evolved such rich flora and fauna it has become known as the Galapagos of the North. Among its wildlife: an estimated 1.5 million seabirds, including one-half of the global population of Ancient Murrelets. But a relatively new invasive species – rats – have been swarming through nesting colonies, necessitating the B.C. project that is part of a growing trend to take back key sea-bird islands from the voracious vermin.

In some places, rats have wiped out hundreds of thousands of birds. But although wildlife managers were doing eradication projects as early as 1961, it wasn't until the early '90s that they started using helicopters for the "aerial broadcast" of baits, a technique that has made it possible to tackle large, rugged and remote islands like those in Haida Gwaii.

On Rat Island in Alaska, the team under the direction of the U.S. Fish and Wildlife Service followed overlapping GPS tracks to shower the area with grain pellets laced with rodenticide in 2008. They would learn later that they had overdosed the island, spreading more poison than was needed. But the results, rat-wise, were dramatic. Within months, Rat Island, which became infested following a shipwreck 230 years earlier, was rat-free. Now, five years later, the island has become a case study on how and why to eradicate rats to save Aleutian cackling geese, ptarmigan, peregrine falcons, black oystercatchers and other species.

Parks Canada is hoping for similar results on Murchison and Faraday islands in the Haida Gwaii archipelago where rats, which came ashore from marine vessels in the 1700s, have been devastating sea bird colonies.

"They are helpless," Laurie Wein, manager of the Parks Canada project, said of the ground-nesting sea birds. "Those birds are just sitting ducks. It's a free-for-all. It's a buffet for rats. They'll eat the eyes, the soft fleshy bits and just leave the carcass."

Officials are hoping to avoid the mistakes made at Rat Island, where the collateral damage was surprisingly high, including 43 dead eagles.

"The issue hasn't been if it's a good or bad thing to do. It's good. But if you don't do it right, it can cause problems," said Ellen Paul, executive director of the Ornithological Council, a U.S.-based non-profit whose analysis of the Alaska project revealed flaws and helped refine rat eradication techniques. The dose per hectare and the seasonal timing are now more carefully calibrated. "On Rat Island, they really didn't know what was out there and when … they assumed during the salmon spawning season the eagles would leave," she said. In fact, they didn't: The eagles fed on dead or dying rats and got poisoned too.

But Ms. Wein of Parks Canada said the Haida Gwaii assault – carried out with Coastal Conservation and Island Conservation, two non-profits long engaged in a global war on rats – appears to be going perfectly.

"This is not a project we entered into lightly. We did two years of pre-planning," Ms. Wein said. Early signs are that few species other than rats are being killed in the attack, in which a helicopter bombarded the two islands and a support crew placed baits by hand on small, adjacent rocky islets.

"There were tens of thousands of rats, potentially hundreds of thousands of rats on these islands," Ms. Wein said. She said it appears all the rats are now dead, or soon will be. "We have to monitor these islands for a couple of years. But things look really good at this point. We're quite confident this will work on these islands," she said, adding other islands may be treated in the future.

Ms. Wein said rats had devastated the sea bird colonies on the two islands and were threatening to spread to nearby Ramsay Island, which is considered "the Crown jewel" in Gwaii Haanas National Park Reserve.

Gregg Howald, North American regional director of Island Conservation, said the Haida Gwaii islands are on a list of about 500 worldwide that have been identified as prime targets for rat eradication projects. He said rats can dramatically change an ecosystem; in the Aleutian Islands, researchers trying to determine just how many birds rats can kill have found up to 30 dead Ancient Murrelets in a single rat burrow.

"In the Aleutians you get these huge colonies, millions of birds … and you hear this cacophony of song, of birds chattering and sometimes there are clouds of birds so thick they almost darken the sky," Mr. Howald said. But when he visited Rat Island before the eradication, he was struck by the silence. No birds sang. "All you heard was the wind. It was an eerily quiet island and all you saw were the rat trails."

Mr. Howald said the best tactic is to use helicopters because that allows an entire landscape to be covered with poison. "A single pregnant female can theoretically repopulate an entire island," he said. "This [technique] is designed to get the last rat. It's not designed to get a rat – it's designed to get the last one."

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