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After being nearly wiped out by industrial chemicals and pesticides, the bald eagle is making a big comeback in Southern Ontario.

Nowhere is the recovery of the majestic bird of prey more remarkable and more symbolic than along the Niagara River, once one of the world's most infamous toxic hot spots. Eagles appear ready to recolonize the area after an absence of nearly half a century.

Starting in the middle of February, as many as seven, including a mature adult male and a female nearing sexual maturity, have been sighted around Navy Island in the Niagara River, just upstream from the Canadian side of the falls.

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Three years ago, provincial biologists erected a nesting platform on the uninhabited island, hoping that eagles would again breed at the site, something that hasn't happened since the mid-1950s. There were no sightings until now, and the fact that so many eagles have descended on it at once has created a huge flap among ornithologists and wildlife experts.

The revival of the endangered bird -- important as a top-of-the-food-chain predator -- is being taken as a hopeful sign that the Great Lakes ecosystem may return to health.

"The good news is the bald eagle is recovering and that means our environment is improving," Pud Hunter, a biologist with the Ministry of Natural Resources, said. "It's certainly a positive indicator."

"It's a good sign. It's nice to see a good sign for a change," said Anne Yagi, another ministry biologist active in the eagle recovery program.

The return of eagles is occurring across most of Southern Ontario, ministry data suggest.

The nadir for eagles was in 1980, when there were only three nesting pairs in all of Southern Ontario, all on the north shore of Lake Erie between Rondeau and Point Pelee. That year, none of the three pairs succeeded in reproducing.

While the species continued to hold its own elsewhere in Canada -- eagles remained common in areas such as the Pacific Coast and Northern Ontario -- it was on the verge of vanishing in Southern Ontario. Last year there were 20 active nests in Southern Ontario, with a total of 28 young, according to Mr. Hunter, who keeps eagle statistics for the ministry.

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As eagles become more common in a territory, population pressure drives the young to seek out new nesting sites, something that probably accounts for their arrival on the Niagara River.

The birds are now found along the St. Lawrence River near the Thousand Islands Bridge, on the Thames River near London, on the Grand River and along Lakes Erie, St. Clair and Huron, among other areas. About the only part of their traditional range where eagles have not returned is the Canadian shore of Lake Ontario, but last year there was one pair on the U.S. side.

Naturalists believe there were about 200 nesting pairs in Southern Ontario around 1900, a population that held its own until about 1945.

The population decline began as industrial chemicals dispersed in the environment, starting in the mid-1940s, with the release of large amounts of polychlorinated biphenyls, the notorious PCBs that were widely used in electrical equipment. The insecticide DDT added insult to injury when its use became widespread in the 1950s.

By the 1970s, adult eagles were laced with high levels of both compounds, absorbed from their prey. The adult birds were transferring the chemicals to their eggs at levels so high that their young were being poisoned in their shells or were so debilitated that they died as eaglets.

The thinning of shells also caused some eggs to crack under the weight of the parent birds, killing the young before they even hatched.

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"The population decline was due to an impairment of reproduction that was due to pollution," Mr. Hunter said.

Both DDT and PCBs are fat-soluble and easily impregnate living tissues. Because they are held in fat, concentrations rise, or bioaccumulate, at each link in the food chain. Levels become extreme in top predators, such as eagles, and even higher in the fatty tissues of their eggs.

At the worst, biologists recorded PCBs at up to 225 parts per million and DDT at 30 ppm in eagle yolks. DDT at 15 ppm is the threshold that causes complete reproductive failure in eagles, Mr. Hunter says.

Since the chemicals have been restricted, DDT in 1978 and PCBs in 1980, levels of both in eagles have fallen by more than two-thirds, and the birds have begun to hatch again. Early in the 1980s, while contaminant levels were still high, many eaglets had gross birth defects, such as crossed bills, that consigned them to premature death.

Although the birds are recovering, biologists warn the revival isn't a return to full health. One concern is that Ontario eagles now live for about 15 years, about half the normal life span. Biologists are also worried by such substances as mercury. Nonetheless, the return of the eagles to the Niagara site is being savoured.

The arrival was first noted on the Canadian side of the river by Bob Chambers, a bird watcher who helped raise funds for the nesting platform and remembers eagles along the river as a teenager in the 1950s. He is thrilled by the return and said recently while watching for the birds that the near disappearance of the species should be a lesson for humans about the need to always keep pollutants in check. "We let them down once. Let's not do it again."

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