While shooting, trapping, and poisoning took their toll, human population growth and land-clearing along navigable rivers and estuaries destroyed prime eagle habitat. Before European settlement, 250,000 to 500,000 bald eagles ranged across North America, and as late as the mid-1800s, wintering eagles reportedly fished the waters off New York's Manhattan Island by the hundreds, sometimes devouring their catch in Central Park.

"The relationship between human development and the absence of bald eagles has been documented in various places across the country," said David A. Buehler, author of the bald eagle monograph in the recently published Birds of North America: Life Histories for the 21st Century.

"In general," Buehler added, "eagles avoid developed areas, where their risk of mortality rises. Shooting, trapping, poisoning, collisions with man-made structures, scarcity of prey, and poor nesting and roosting habitat are among the dangers. I think it was the human persecution, however, that ultimately 'taught' eagles in an adaptive sense to avoid people."

With the westward expansion of human settlements, persecution and habitat destruction whittled away at eagle numbers. By 1940, the bird's rarity compelled Congress to pass the Bald Eagle Protection Act, which outlawed the killing and disturbing of eagles, as well as the possession of eagle parts, including feathers, eggs, and nests.

After studies showed that salmon populations were not harmed by eagle predation, this law ended a bounty system in Alaska that claimed 128,000 eagles between 1917 and 1952. The actual number of slaughtered eagles probably exceeded 150,000, since many bounties were never collected.

For a long time, the Bald Eagle Protection Act, designed also to protect the beleaguered golden eagle, was not strictly enforced. At one Wyoming ranch, for example, eagles were systematically shot for their perceived threat to livestock. According to a 1970 report, more than 770 bald eagles were shot at this ranch, and hunters were paid $25 for each carcass. Responding to a public outcry over such flagrant violations, the government began to crack down.

Just when it was finally benefiting from legal protections, the bald eagle took a heavy blow from DDT, a pesticide that enters the food chain and causes reproductive failure. Widely used after World War II to control mosquitoes and other insects, DDT was wreaking havoc among many bird species. Raptors were particularly vulnerable--over time, animals higher in the food chain accumulate more DDT.

New research on the effects of DDT challenges the long-held belief that eggshell thinning was the primary cause of reproductive failure in birds. "The thinning did occur," said Buehler, "but it was probably not actually responsible for the reproductive failure."