For many North Americans, loons are a much-beloved bird, symbolizing the solitude of the deep-woods wilderness with their distinctive, haunting wail that echoes over the northern lakes where they breed in summertime.
But about 30 years ago, people noticed shrinking numbers of the common loon—a stately black-headed, black-and-white checkered bird—in parts of Northeastern North America. Researchers began capturing loons on their breeding lakes in Southeast Canada in the mid-1990s to study them and monitor reproduction—and discovered high levels of mercury in the birds' blood and feathers.
Additional research showed that birds born in New York State and New England also were being exposed to large quantities of methylmercury, the form of mercury toxic to living things. After winter migration, loons return each year to their birthplace to nest—and these lakes were poisoned.
"The mercury levels in common loons in that part of North America are probably some of the highest levels in living animals anywhere in the world," said Mark Pokras, a veterinarian who runs the wildlife clinic at Tufts University Veterinary School in North Grafton, Massachusetts.
Mercury Alarm
This discovery raised an alarm: pollution in northern lakes posed a significant threat to fish-eating animals—and to people. It also added another dimension to the responsibility of U.S. and Canadian government regulatory agencies.
Now common loons, one of five loon species, are helping scientists better understand the impact of environmental mercury contamination on waterbirds, fish, and other aquatic wildlife. The birds are particularly vulnerable to environmental poisoning for many reasons. They are long-lived—up to 30 years—and they spend their lives in the water, feeding mostly on fish.
"Loons are at the top of the food chain, so they are an excellent indicator of environmental quality," said Nina Schoch, program coordinator for the Adirondack Cooperative Loon Program (ACLP) in Ray Brook, New York. "They are also an extremely charismatic species: people care deeply about them and are concerned about [their welfare]," she said.
These studies have also led to widespread warning for anglers about eating fish from affected regions.
Mercury on the Wind
Most of the mercury pollution that reaches northern lakes is spewed into the atmosphere by large coal-burning power plants and municipal waste incinerators in the Midwest and central Canada.
Wind currents carry mercury hundreds of miles eastward, along with compounds that create acid rain. The pollutants fall to earth in snow, rain, and dust particles, eventually washing into the many lakes and ponds that dot the region.
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