The peregrine falcon was once the terror of American skies-- diving on other birds at 200 miles per hour, striking them with clenched talons, and knocking their shattered corpses out of the air--and now, according to the U.S. Fish and Wildlife Service, it can be declared officially back. DDT, that insidious thinner of eggshells, had nearly done the peregrine in. By 1970 there were fewer than 50 pairs of falcons in the whole country, all west of the Rockies, says Jeff Cilek of the Peregrine Fund. After DDT was banned in 1972, the Peregrine Fund and other conservation groups began breeding peregrine falcons in captivity and releasing young birds at carefully selected cliffs. There are now about 1,000 pairs of peregrines in the lower 48 states, not a few of which are nesting on skyscrapers rather than cliffs; New York alone has 15 pairs. Last June the Fish and Wildlife Service proposed that peregrines be removed from the endangered species list.