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Feature Story
Empty Nest Syndrome
Page 3

Oh, Rats

The battle against the black rat preoccupies many of the biologists fighting to protect critically endangered birds in Hawaii. Some have used rat poison sealed in bait stations -- specially designed boxes that only a rat is likely to enter. These bait stations can reduce rat populations while keeping other animals safe from the poison, but they must be continually checked and refilled, a labor-intensive task in the steep, slippery terrain of Hawaii's high-elevation rainforests. And the job never ends, because new rats make their way into Hakalau and other refuges from unprotected parts of the islands. There simply isn't enough money from the state and federal governments to make this a practical method of rat control. And Hakalau, at more than 30,000 acres, is too big an area for bait stations to offer realistic protection.

Using an infrared viewer by night, Jeffrey has watched the black rats moving high in the forest canopy. He's seen the nests of palila, an endangered honeycreeper that lives up-mountain from the refuge, devastated overnight, with only bones, eggshells, and a few telltale rat droppings left behind. Researchers studying the puaiohi, a small thrush that survives in a few remote drainages on Kauai, have seen healthy females and chicks disappear, then found their bones at the entrances to rat dens. In the last stronghold of the endangered Oahu elepaio, a perky little bird related to mainland flycatchers, scientists used cameras wired to artificial nests to take mug shots of predators. In every case the culprit was a black rat. After a baiting program reduced the rat population, the survival of the nests shot up from less than 40 percent to 80 percent.

Aki nests are too few and far between to make direct study practical, but in light of these other findings, it's a safe bet that rats are preying on eggs, chicks, and adult females. "A female aki raises one chick a year, sometimes only one every other year," says Jeffrey. "And you're looking at a population of maybe 1,000 birds. When a rat strikes a nest, you're losing a lot."

Jeffrey helped identify Hakalau as critical habitat back in the late 1970s, when it was still managed as a ranch. He has worked full-time at the refuge since 1990, spending long hours walking the steep terrain and monitoring its bird life. Strange as it may seem, given his meticulous efforts to protect wild nature here, he hopes to see rat poison dropped from the sky onto the haven that's been his life's work.

"It's a real possibility that aerial drops of rodenticide are the only thing that's going to save some native bird species," Jeffrey says. "But before you can use it you have to jump through all the regulatory hoops. And then there's public reaction: When the government says, 'We're going to drop this poison all over a watershed,' how do you think people feel about that?"

Hawaiians who carry on the old-time tradition of hunting feral pigs are especially concerned about the prospect of aerial drops of rat poison; they worry that pigs might eat the baits, tainting their meat, or that hunting dogs might succumb to the toxin.

The key ingredient in the rat baits is diphacinone, a blood thinner that was once used to treat cardiac patients. A high dose can cause fatal internal bleeding; rats are far more susceptible to the chemical than are most other mammals or birds. In laboratory tests, it takes as little as 2.3 milligrams of diphacinone per kilogram of body weight to kill a rat. The deadly dose for a pig is much higher -- 150 mg/kg -- while a mallard duck, the laboratory stand-in for birds of all kinds, can handle up to 3,000 mg/kg.

Catherine Swift, a biologist with the U.S. Fish and Wildlife Service in Honolulu, has spent years field-testing diphacinone baits, monitoring the animals that feed on them in the wild, to find the lowest possible dose that will effectively kill rats. Based on studies done with placebo baits, Swift expected that rats would quickly devour the poison pellets, with little risk that other, nontargeted animals would be affected.

When I first spoke with her, in 2002, Swift felt that the main barrier to aerial rodenticide drops was one of public education. Diphacinone, she pointed out, is widely used to control rats in farm fields and is less likely to harm a hunter's dog than the kinds of rodent poison sold in most hardware stores. She and many of her colleagues hoped that airdrops of diphacinone baits would be the silver bullet that stopped rats from destroying not only native birds but endangered plants like the 'Oha wai (Clermontia pyrularia and lindseyana) and a species of haha (Cyanea shipmanii), which can't survive repeated gnawing by hungry rodents.

Erik Tweed, a biologist with the U.S. Geological Survey, was less confident. Tweed spent two years, from 1998 to 2000, in the steep, rain-drenched Alakai Swamp on the island of Kauai, tracking 14 captive-reared puaiohi -- part of a remaining band of about 200 to 300 birds -- that were released back into the wild. "By far the single largest cause of nest failure is rat predation," he says. "If we could control rats effectively, I don't see any reason why puaiohi nests wouldn't be 80 to 90 percent successful. But if you do anything on a large scale, like dropping rat baits by air, you don't know what the consequences are going to be."

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Map: Small World Maps

OnEarth. Summer 2005
Copyright 2005 by the Natural Resources Defense Council