When Wisconsin eagles and ospreys didn”t reproduce normally in the 1960s and 1970s, wildlife biologists identified the culprit – high levels of chemical contaminants in the environment and eggs.
Although the birds are making a recovery, Wisconsin biologists recently checked two populations where they anticipated problems. Eagles along the state”s Lake Superior shore raise 23 percent fewer chicks per nest than eagles nesting along Wisconsin”s inland lakes and rivers. The Lake Superior nestlings also have six times more DDE and nearly four times more PCBs in their blood than eaglets from inland nests. Ospreys that nest along the Castle Rock and Petenwell flowages in central Wisconsin lay eggs with dioxin concentrations that have been reported to inhibit reproduction in some other birds.
Are contaminants still hampering the birds” nesting success?
Two studies published this May and July indicate DDE, PCBs and dioxin in the environment aren”t a major problem for the Lake Superior bald eagles or Wisconsin River ospreys. According to a University of Wisconsin-Madison scientist, the research points to food limitation for Lake Superior eagles and finds that osprey raise young successfully despite dioxin in the Wisconsin River.
“Those findings should be seen as good news,” says William Karasov, a wildlife ecologist with the College of Agricultural and Life Sciences. “Wisconsin has been a leader in improving water quality. State and federal efforts appear to be paying off.”
Karasov, graduate students Cheryl Dykstra and James Woodford from the UW-Madison Department of Wildlife Ecology, and Michael Meyer, a wildlife biologist with the Wisconsin Department of Natural Resources worked on the projects. The eagle study also involved scientists from Michigan State University and the University of Minnesota.
Despite the encouraging findings, Karasov and Meyer offer several cautionary notes.
* First, contaminant levels in other parts of the state, Green Bay for example, remain high enough to reduce eagle reproduction.
* Secondly, animals differ greatly in their sensitivity to contaminants. Although ospreys along the Wisconsin River may tolerate the dioxin there, it may be too great for other wildlife.
* And finally, the two studies don”t address the toxins” long-term effects on bird growth and reproduction. Many biologists think some contaminants disrupt hormones that regulate reproduction and other key functions.
DDE, a metabolite of the insecticide DDT, was largely responsible for a steady drop in bald eagle numbers between 1940 and 1970, Karasov says. From 1970 to 1976, no eagles nesting on Lake Superior produced young.
After DDT and similar insecticides were banned, the American bald eagle population rebounded quickly. The number of bald eagle pairs nesting in Wisconsin, for example, has risen from 108 In 1973 to 645 in 1997. However, eagle populations along the Great Lakes have not kept pace and reproduction there lags that of eagles nesting on inland waters.
Between 1986 and 1994, the researchers monitored chemical contaminants in eagle eggs and nestlings, and the food delivered to 15 eagle nests along Lake Superior and 38 nests from inland lakes and rivers of northern and central Wisconsin.
“The evidence indicates that DDE is no longer responsible for low bald eagle productivity on Lake Superior,” says Karasov. “Although the eagle eggs and young there had more DDE than they did at inland sites, the DDE concentration in eggs was well below the levels at which we expect it to cause reproduction problems. If it were causing the problem, you”d expect to see the eggs not hatch rather than the young die as partly grown nestlings. The timing of death, when nestling food demands are greatest, indicates a cause other than contaminants.”
The ecologists found that parent eagles on Lake Superior delivered less than half as much food to their offspring as eagle parents inland. Nestling mortality was three times greater on Lake Superior than inland waters. Most of that mortality occurred where parents were trying to rear two rather than a single eaglet. Individual nestlings in these broods got even less food than those where there was only one nestling to feed.
“Reproduction of eagles on Lake Superior may have stabilized at a naturally low level, perhaps because the lake is simply marginal fishing habitat for nesting bald eagles,” Karasov suggests. “It”s a deep, cold, rather unproductive lake with few shoals where fish can spawn and eagles can forage successfully.”
PCBs may contribute to the problem. “We don”t know the critical level of PCBs in eggs that might depress reproduction or the specific types of PCBs from Lake Superior,” Karasov says. “However, eagles nesting on the Wisconsin River have total PCB levels nearly the same as those on Lake Superior, yet have higher reproductive success.”
The Castle Rock and Petenwell flowages are contaminated with dioxin as a result of manufacturing processes formerly used by two pulp mills upstream of the flowages. While the dioxin discharges have been virtually eliminated, the contaminant persists in sediment and fish. The fish that osprey eat there have 30 to 100 times higher levels of dioxin than fish upstream in two other flowages on the Wisconsin River – the Rainbow Flowage and the Mead Wildlife Area. Two published studies indicated that contaminant levels like those at the Wisconsin site would impair osprey reproduction.
Karasov says the situation was an opportunity to conduct an experiment to see if dioxin were a problem and to separate out confounding factors that might depress reproduction, such as parental behavior, food abundance and predation on the young. Osprey, unlike eagles, aren”t an endangered species. Therefore, the researchers could move entire clutches of eggs from the Castle Rock and Petenwell flowages north and place them in osprey nests in less polluted environments, and also make the reverse exchange.
The researchers collected and analyzed the contaminants in osprey eggs between 1992 and 1996. They switched eggs among nests during 1995 and 1996.
“Although eggs from the Castle Rock and Petenwell flowages were significantly more contaminated than eggs from the upstream sites, we did not find that exposure to dioxins, in the egg or through the food chicks ate after they hatched, affected the survival of osprey chicks,” Karasov says.
However, nestlings exposed to the higher level of contamination did grow more slowly than osprey chicks from the upstream sites. The researchers ruled out food as the cause of the slower growth because osprey parents on the Castle Rock and Petenwell flowages actually delivered more food to their chicks than did the osprey parents at the upstream sites. The researchers say dioxin may be responsible for the slower growth rate of osprey chicks at the contaminated site.
The Great Lakes Protection Fund, the Wisconsin Department of Natural Resources, the National Park Service, the U.S. Fish and Wildlife Service, the Zoological Society of Milwaukee County, and the Wisconsin Society for Ornithology provided major support for the studies.