听力原文： America's national symbol—the Bald Eagle—almost went extinct 20 years ago. But it has made a comeback In fact, the US Fish and Wildlife Service is considering the possibility of taking it off the endangered species list. Once, more than 50 000 pairs of bald eagles nested across the country. But by 1960, that number had fallen below 400. The chief killer was the widely used DDT. Fish soaked up DDT died and were washed up on shores where bald eagles feasted on them. (29)DDT prevented eagle eggshells from thickening. The shells became so thin that they shattered before the babies hatched. Fortunately, in 1972, a law was passed to ban DDT. which saved the bald eagle from total wipeout. (30)And since then, wildlife biologists have reintroduced bald eagle eggs from Canada to America. The result was that last year US bird watchers counted 11 610 bald eagles in the country. If it were dropped from the endangered list, the bald eagle would still be a threatened species. That means the bird would continue to get the same protection—no hunting allowed, and no disturbing of nests. (31)But bald eagles still face tough times. The destruction of their natural homes could be the next DDT causing eagle numbers to drop quickly. (30)