It could help explain the high levels of such pollutants found in the bodies of people living in and near the Arctic region, far from the industries that produce them.

The birds eat fish, squid and other animals that concentrate the chemicals in their bodies. The chemicals are concentrated even more in the bodies of the birds.

The birds, some of which range for thousands of miles (kilometer), then rain polluted guano onto once-pristine environments, Jules Blais of the University of Ottawa and colleagues found.

"The effect is to elevate concentrations of pollutants such as mercury and DDT to as much as 60 times that of areas not influenced by seabird populations," said John Smol, a biology professor at Queen's University in Kingston, Ontario.

"Our study shows that sea birds, which feed in the ocean but then come back to land, are returning not only with food for their young but with contaminants as well. The contaminants accumulate in their bodies and are released on land."

Blais and colleagues studied ponds below cliffs on Canada's Devon Island near Greenland, where a large colony of petrels called northern fulmars live.

The ponds right underneath where the birds roost contained higher levels than nearby sites of DDT, mercury and hexachlorobenzene, used in pesticides.

Fulmars range as far as 250 miles (400 km). But other species of birds are known to range much farther -- terns, for instance, fly back and forth from the Arctic to the Antarctic.

Bird droppings end up on the land and back in the sea, where they join the food web. Pollutants in them could be taken up by plants and animals that are eaten by larger animals such as seals, polar bears, fish and whales.

Some of these animals in turn are eaten by people.

"Some chemicals will build up in the food webs that comprise northern traditional diets," Linda Kimpe of the University of Ottawa said in a statement.

"As a result, some of our northern Canadian populations are among the most mercury and PCB-exposed people on the globe."

Mercury, released by coal-burning power plants, and PCBs, which were once produced by industry, build up in body fat and thus are found at especially high levels in fatty animals such as whales and seals and fish such as tuna and salmon.

"Most of Canada's coastline is at our northern fringe, and northern aboriginal communities rely on these ecosystems as a source of nutrition, economic development, traditional customs, and culture," Blais said in a statement.