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.
The Midwest launched its own release programs in the 1980s under the coordination of the Raptor Center in Minnesota. The Chicago Peregrine Program began in 1985 as a cooperative effort between the Chicago Academy of Sciences, Lincoln Park Zoo, Illinois Department of Conservation, and Illinois Audubon Society. From 1986-1990, the Peregrine Program released a total of 46 Peregrines from four different hack sites.
The goal was to help re-establish Peregrines on a regional basis in the Midwest, with the hope that some birds would return to Illinois to breed. This regional falcon dispersal can be seen through the identity of our adult Peregrines. Over the years, Illinois-based birds have originated from nest sites located in Indiana, Minnesota, Iowa, and Wisconsin. When population levels in the Midwest finally began to rise, the focus of Peregrine programs shifted from releasing birds to monitoring nesting pairs. Illinois has now progressed from having only a single breeding pair at the Chicago-Wacker site in 1988, to 12 breeding pairs in over 23 different territories by 2011.