Arctic and American peregrine falcons (Falco peregrinus tundrius and F. p. anatum, respectively) were listed as endangered in 1970. At the time, some local populations of American peregrine falcons in the eastern United States had disappeared, and populations in western and northern North America had been reduced by 80 percent or more. Organochlorine pesticides such as DDT and its breakdown product DDE were identified as the main cause of the decline. The peregrines accumulated these chemicals in their tissues by feeding on birds that had eaten DDT-contaminated insects or seeds. These chemicals prevented normal calcium deposition during eggshell formation, and caused females to lay thin-shelled eggs that often broke before hatching. The use of DDT was restricted in the United States and Canada in the early 1970's, and populations of peregrine falcons in North America began to recover by the late 1970's.
After Arctic and American peregrine falcons were listed, the Fish and Wildlife Service prepared recovery plans for four different geographic areas. For Alaska populations, the recovery plan identified specific "index" areas (areas representative of interior and northern Alaska) to survey and specific recovery criteria for reclassification. These criteria included the number of pairs occupying territories, number of young produced, reductions in DDE residue in eggs, and minimum eggshell thickness.
In the early 1980's, biologists in the Service's Region 7 Endangered Species and Environmental Contaminant programs began a contaminant monitoring program for peregrine falcons in Alaska. This program continued throughout the 1990's. The monitoring plan focused on DDE and eggshell thinning, and called for collecting and analyzing at least 10 eggs from each subspecies every 5 years. Unhatched eggs were also collected when visiting nests to band falcons for mortality and movement studies. We began the program in 1984 and repeated it in 1989 and 1995. During this time, we collected 153 eggs, 87 from American peregrines and 66 from Arctic peregrines.
Our analyses showed a clear downward trend of DDE concentrations in eggs. In the late 1960's, DDE residues in the range of 20-40 parts per million (ppm) and eggshell thinning in excess of 20 percent were observed for peregrine falcons in Alaska (Peakall et. al 1975). …