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Pollution, Food Stress Take Toll On Bald Eagles

June 8, 1998
Simon Fraser University
The work of a Simon Fraser University graduate student may have helped solve a bald eagle mystery on Vancouver Island.

The work of a Simon Fraser University graduate student may have helped solve a bald eagle mystery on Vancouver Island.

Chris Gill, a master's student in biological sciences, has spent the last two summers studying bald eagles near Duncan, B.C. in an effort to find out why an alarmingly high number of nests are failing to produce healthy chicks.

The area of concern lies just south of the Crofton pulp and paper mill, and is part of a zone closed to fisheries in the 1980s due to high levels of dioxins -- toxic byproducts of the pulp bleaching process. Although the mill significantly reduced its dioxin output about 10 years ago, the bald eagles have failed to rebound. A 1995 study showed that three out of four nests in the closure zone failed, compared to an almost 100 per cent success rate for nests outside the zone.

"'Failed' means that there were eggs but they didn't hatch, or there were chicks but they died," says Gill. Dioxins seemed the most likely culprit, he says, yet that same 1995 study didn't find similar patterns near other coastal pulp mills. "This suggested that Crofton was different, but we didn't know why."

Gill looked to the eagles themselves for answers. He worked at two sites -- one near the mill, and the other, for comparison, on the west side of Vancouver Island near Bamfield. Over two summers he captured 24 eagles -- 10 adults and 14 eaglets -- and took blood samples to assess levels of pollutants such as dioxins, PCBs and DDEs (a form of DDT). He conducted aerial surveys by helicopter, counting the number of incubating birds and the number of nestlings produced. And he watched eagle nests, from dawn to dusk, for a total of 850 hours.

He also filmed eagle family life. With the help of a professional tree climber -- eagle nests are typically 30-40 metres off the ground -- Gill mounted a video camera above a nest and programmed a battery-powered VCR at the base of the tree to record nesting activity. He later analysed a total of 1,100 hours of videotape.

He counted prey deliveries, the size and type of prey, adult and chick behavior, and how long adults stayed at the nest -- a good measure of how easy it was to find food. "Looking at contaminants was an obvious place to start the study," Gill says, "but we had to consider the possibility that food availability was responsible. We couldn't look at just one thing and expect to find all the answers."

Sure enough, the results suggest that contaminants and food stress are both contributing to the eagles' difficulties, particularly south of the mill where the tide sweeps effluents into a large, reservoir-like bay. "All the contaminants were six to 15 times higher in eaglets south of the mill," says Gill. "North of the mill, contaminants were much lower and nesting success was about 20 per cent higher."

Each contaminant detected in birds south of the mill was below the level estimated to cause lower nesting success. But those estimates don't take into consideration the effects of multiple contaminants, or food stress, says Gill.

"South of the mill, there's a very deep channel which makes it much harder for the birds to catch fish," he explains. Because eagles are territorial -- adults will vigorously chase away intruders -- some nesting pairs get stuck with the poorer fishing areas.

Exactly how contaminants are getting into the Crofton eagle chicks is not known, says Gill, although fish seem the obvious source. More research is planned for this summer. In the meantime, the good news is that contaminant levels from the pulp mills are dropping as time goes on. "There's basically little else that pulp mills can do," says Gill, who sees his study as a valuable baseline for future research on pollution in eagles or any other bird of prey.

"People love bald eagles," says Gill, who still stops to watch whenever he sees one. "It's like killer whales; we don't want to see anything happen to them because of something we've done to the environment that we have the power to fix."

Gill's study was funded by the Canadian Wildlife Service and the Bald Eagle Rescue and Research Foundation.

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Materials provided by Simon Fraser University. Note: Content may be edited for style and length.

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Simon Fraser University. "Pollution, Food Stress Take Toll On Bald Eagles." ScienceDaily. ScienceDaily, 8 June 1998. <>.
Simon Fraser University. (1998, June 8). Pollution, Food Stress Take Toll On Bald Eagles. ScienceDaily. Retrieved April 14, 2024 from
Simon Fraser University. "Pollution, Food Stress Take Toll On Bald Eagles." ScienceDaily. (accessed April 14, 2024).

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