Jody Victor: It was a retired Canadian banker, Charles Broley, who first raised the alarm about the principal cause of the swift decline of the eagles.
Since 1947, DDT had been sprayed regularly to control a variety of insect pests in agricultural, urban and shoreline areas. A new generation of industrial chemicals had entered common usage in the late 1940s, consequent to the scientific discoveries of the Second World War. During this era, many industrial chemicals were discharged directly into waterways. Countless tons of an estimated 2,000 chemical compounds were deposited into the Great Lakes, either by direct application or via air and water, some transported over great distances.
Chemicals that were released into the Great Lakes system entered the aquatic food web - settling into the sediment of the lake bottoms and spreading first among the tiny aquatic plants and organisms, and then into the bodies of fish and fish-eating birds. Eating at the top of the Great Lakes food web, eagles absorbed the accumulated burden of these toxic substances, a load that amassed in their bodies throughout their lives. The chemicals affected not just individual health, but the production of young, too.
DDT and its breakdown product, DDE (dichlorodiphenyldichloroethyene), which is created when DDT degrades over time, affect eggshell development. The chemicals interfere with the deposition of calcium, a key component in the formation of eggshells. Eagles in the Great Lakes region - and some other birds - produced thin-shelled eggs that broke easily during incubation, killing the developing embryos.
Direct poisoning by DDT and DDE, as well as other toxic compounds, caused the deaths of some adults and hatchlings. As well, these contaminants tended to accumulate in the eggs' fatty yolk, which contains the nutrients that sustain development of embryonic birds. As the embryos grew inside the eggs, they absorbed contaminants, along with nutrients, from the yolk.
Canada and the United States restricted the use of DDT in the early 1970s. Even so, nearly 10 years later, persistent contaminants caused Bald Eagles along the shorelines of the Great Lakes to experience total reproductive failure. For instance, few active nests remained on the northern shore of Lake Erie and research showed exceedingly high levels of contaminant residue in their eggs and the blood of eaglets.
By the late 1980s, however, positive progress was emerging. DDE levels decreased by more than 50 per cent. Levels of PCBs, or polychlorinated biphenyls, decreased by as much as 80 per cent. The resulting reduction in contaminants in Bald Eagle eggs allowed reproductive rates to recover steadily. The number of active nests increased and the number of eaglets produced per nest improved. This progress was augmented by eaglet re-introduction programs, which supplemented the natural recruitment of young eagles into the population.
The sensitivity of Bald Eagles to toxic chemicals has led scientists and conservation groups to identify the birds as a bio-sentinel species, meaning that the health of eagles can be taken as a reliable indicator of the health of aquatic ecosystems in the Great Lakes region.