Decline and Recovery Historically, an estimated 400-500 pairs of Peregrines once nested in the Midwest and eastern United States. But by the 1960s, the species had been extirpated (wiped out regionally) and few were seen during migration. The primary cause was the buildup of organo-chlorines—DDT and its byproducts—in the birds. These accumulated chemicals caused addling of eggs, abnormal reproductive behavior in adults, and thinning of shells, which led to egg breakage. In 1972, the government banned the use of DDT in the United States, and a year later, placed the Peregrine Falcon on the Federal Endangered Species List. During this same time period, the Peregrine Fund was established. It oversaw the reintroduction of Peregrines in the east using a process called hacking, a delicate method of releasing birds back into the wild. (View our Photo Gallery to see how this method works.) continue >>