ROSE LAKE, Idaho -- A grim death toll of migrating tundra swans is again being observed at northern Idaho marshes contaminated with toxic mining waste.
Thousands of swans headed for breeding grounds in Alaska stop each spring at the marshes along the Coeur d'Alene River.
But the roots and tubers they feed on are laced with lead that's part of about 100 million tons of mining waste from the Silver Valley that has washed into the river system over the past century.
Lead shuts down the swans' digestive systems and the birds slowly starve to death -- at least 150 annually.
"For me, it's like bearing witness," Kate Healy, a biologist with the U.S. Fish and Wildlife Service, told The Spokesman-Review. "They die slow, agonizing deaths."
Strobel Marsh is among the most deadly for swans, with a lead concentration level 10 times higher than the safe limit for waterfowl.
The U.S. Environmental Protection Agency is working to clean up the Coeur d'Alene Basin to make it safer for humans and wildlife, and expects to spend $360 million over the next 25 years on the effort.
The Fish and Wildlife Service estimates that 92 percent of swan deaths along the river are due to lead. About 2,000 to 6,000 tundra swans feed along the river annually.
Besides swans, researchers said they have documented lead exposure and health problems in at least 26 other wildlife species in the basin. Those include beavers, screech owls, field mice, wood ducks, robins and song sparrows.
Roy Brazzle, a biology technician with the Fish and Wildlife, said it's hard to track the deaths of other species because they don't leave behind "a big white carcass."
Work is being done to create safe feeding areas for swans. In one spot, former hayfields have been flooded to create a new marsh, which has been attracting swans and other waterfowl.
"This is safe feeding," said Healy. "I love to see the swans here."