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THE DECLINE OF THE EAGLE
It is estimated that when the Bald Eagle was adopted as our national symbol in 1782, there were only about 100,000 nesting pairs and reports had already been made of its decline. Many factors have taken a toll on the populations of this magnificent bird of prey—indiscriminate shooting, prejudice, poison baits for killing coyotes and wolves, government bounties, systematic use of the pesticide DDT and dumping of other toxic chemicals, habitat destruction, overfishing (which reduced a significant food source) and lack of education about the eagle’s role in the environment.
DDT AND EGGSHELL THINNING
Populations steadily declined until the 1940s, when the first attempts to help the plight of the eagle were made. The Bald Eagle Act, passed in 1940, provided some protection and gave a glimmer of hope that the eagle would make a return. This bright spot quickly dimmed, however, when it was discovered the effects of DDT were having on Bald Eagles, Common Loons and many other large predatory birds. DDT had been heralded as a miracle chemical for its effective control of insects. What was not understood at the time was the building up of DDT (bioaccumulation), which was how the pesticide occurred in top predators such as eagles.
This is how it worked: DDT was sprayed liberally to control insects. The insects were contaminated, and then eaten by fish. A fish might eat hundreds of insects, all of which had trace amounts of pesticide. As the fish ate more contaminated insects, the amount of DDT increased in its own body. Now enter the Bald Eagle. A large portion of its diet is fish, and most fish had small amounts of pesticide. With each fish it ate, an eagle consumed more and more contaminant, resulting in a large dose of DDT delivered into the eagle’s system (DDT bioaccumulation).
The pesticide didn’t produce external symptoms in adult birds, but it greatly impacted the reproductive system. DDT bioaccumulation caused Bald Eagle eggshells to become thin and fragile—so much so that the weight of incubating adults was causing the shells to break and developing chicks were being killed. Since eagles can live 20 years or more in the wild, the effects of diminished reproduction went unnoticed until after the contaminated adult birds started dying of natural causes. The lack of a younger generation of eagles replacing the aged and dying eagles pointed to the painfully obvious effects of DDT. By then it was nearly too late to save the species. Eagles had been eliminated from many regions and were barely holding onto others.
By the early 1960s, the population of eagles hit its all-time low due to DDT bioaccumulation, with an estimated population of just over 400 nesting pairs in the lower 48 states. At this time Alaska and Canada had relatively robust populations, but these were also impacted and declining. In addition, Bald Eagles in Alaska were being killed by fishermen who thought they were protecting salmon, resulting in a reduction of at least 100,000 more eagles over the past century.
THE RECOVERY OF THE EAGLE
The 1970s were the turning point in the recovery of the Bald Eagle. Two significant issues came to fruition during this decade. The first was the banning of DDT in the United States in 1972. It would take many years for the effects of the pesticide to flush through the natural cycles, but we were well on the road to recovery. The second and, some say, just as important, was the passing of the Endangered Species Act in 1973. This landmark legislation protected the Bald Eagle and many other species in peril. It provided special protection with consequences for violators of the law.
