Peregrine Falcon Pairs in the Midwest, 1999 Mary Hennen and her team carefully track the nesting locations and breeding success of Peregrine pairs in Illinois to monitor population recovery. Notice how much the population grew between 1999 and 2003. Peregrine Falcon Pairs in the Midwest, 2003 Scientists also track the types of nesting sites that Peregrines select. Surprisingly, building ledges are the most popular location, far outranking cliffs, the traditional site.