The recovery of the American peregrine falcon is a success story. Only thirty-nine known pairs of wild peregrine falcons—a large bird of prey—were found in the United States in 1970, and now more than one thousand pairs live in the lower forty-eight states, some nesting on skyscrapers in our largest cities.The bird has been officially designated by the U.S. government as recovered: A bird has been brought back from the brink of extinction.

Such restoration of endangered animals and plant species is the goal of the Endangered Species Act, a law passed by the U.S. Congress in 1973.Yet the act may not have had much to do with the return of the peregrine falcon.How successful the law has been is a subject of great debate...