Severe declines in peregrine falcon numbers began in the 1950s [1]. These declines were linked to organochlorine pesticides that were put into use following World War II, and whose use peaked in the late 1950s-early 1960s [1]. Scientists investigating the peregrine's decline found unusually high concentrations of the pesticide DDT and its breakdown product DDE in peregrine falcons and other birds of prey [2]. Organochlorine pesticides cause direct mortality and reduced reproduction in birds of prey who, being at the top of the food chain, ingest high doses of pesticides concentrated and stored in the fatty tissue of prey animals that themselves ingested contaminated food [1]. Heavily contaminated females may fail to lay eggs and organochlorines passed from the female to the egg can kill the embryo before it hatches. Probably the most serious problem resulted from DDE, the principal metabolite of DDT, which prevents normal calcium deposition during eggshell formation, causing eggs to frequently break before hatching [1]. Arctic peregrine numbers reached their lowest levels in the early 1970s and in some areas of North America successful reproduction virtually ceased [1]. Populations are thought to have decreased by as much as 80% [2].

Following the passage of the Endangered Species Act in 1973, the use of DDT and other organochlorines became severely restricted in the U.S. (their use had been restricted in Canada in 1970) [1]. These restrictions were the most pivotal action in aiding the recovery of the peregrine falcon and Arctic peregrine falcons recovered substantially after organochlorine pesticide use was curtailed [1]. Breeding surveys conducted in widely scattered areas showed that productivity rates returned to normal after the restrictions and as a result, populations expanded. This was particularly true in northern areas, where pesticide exposure was lower and impacts upon populations were less severe [1]. By 1984, the recovery of Arctic peregrine falcons had progressed sufficiently that the USFWS reclassified the subspecies as threatened [2]. The number of Arctic peregrine falcons continued to increase and in 1991, the USFWS began reviewing the Arctic peregrine’s threatened status to determine if a proposal to delist was appropriate [2]. The Arctic peregrine was delisted in 1994.

Four major factors were considered in the delisting process: (1) Population size and trend, (2) reproductive performance, (3) pesticide residues in eggs, and (4) eggshell thickness [1]. Despite a lack of long-term studies using consistent methodologies, there was strong evidence of significant population increases from throughout the Arctic [1]. Four areas in northern North America (one in Alaska and three in Canada’s North West Territories) for which historical survey information was available indicated the number of Arctic peregrine pairs occupying nesting territories increased since the 1960s [1]. Some areas of Alaska even exceeded the original estimates of pre-DDT-era population size [1]. In addition, in the eastern Arctic, peregrines began nesting in previously vacant nesting sites [1]. Standardized yearly migration counts at Cape May, New Jersey, an area where Arctic peregrines concentrate during migration, also saw increasing numbers, most likely from Arctic breeding grounds especially in Greenland and eastern Canada (these counts may have also contained peregrines in the American subspecies; however, banding recoveries indicate that the majority of peregrines along the East Coast during fall migration are from the Arctic and thus represent a true increase in Arctic peregrine numbers) [1].

At the time of delisting, it was determined that reproduction had met recovery goals. Productivity in all regions studied had been sufficient to support a stable or increasing population since the 1980s [1]. There had also been improvements in levels of DDE concentration in eggs. Concentrations in excess of 15-20 ppm (parts per million, wet weight basis) are associated with high rates of nesting failure. Residue in eggs in 1993 was well below the 15-20 ppm critical level [1]. In addition, Alaskan eggshells collected in 1988-1991 averaged only 12% thinner than pre-DDT thickness (17% or greater reduction in thickness results in population declines).

Today, Arctic peregrine numbers continue to increase. On the Sagavanirktok River in Alaska where Arctic peregrine surveys have been conducted since the late 1950s, the number of pairs increased from five in 1958, to 23 in 1992, to 25 in 1999 [3]. Migration counts at the Cape May Hawkwatch site saw an increase from 103 migrating peregrines in 1976, to 429 in 1992, to 1,017 in 2004 [4]. Although the species is currently doing well, there are still threats, such as habitat modification that could potentially affect Arctic peregrines [5]. Since habitat modification has drastically increased since the 1970s, however, while Arctic peregrine populations tripled during the same period, habitat modification may not currently threaten the continued existence of the subspecies [5]. Pesticides accumulated in Latin America, where the use of DDT continues, still affect eggshell thickness, and although shell thickness has increased, it is still below pre-DDT levels and therefore still at risk of decreasing to below critical levels [1].

[1] U.S. Fish and Wildlife Service. 1993. Proposal to Remove the Arctic Peregrine Falcon From the List of Endangered and Threatened Wildlife. Federal Register (58:188).
[2] U.S. Fish and Wildlife Service. 1995. Peregrine falcon, (Falco peregrinus anatum, Falco peregrinus tundrius, Falco peregrinus pealei). Species account. Website (http://www.fws.gov/species/species_accounts/bio_pere.html) accessed October, 2005.
[3] Wright, J.M. and P.J. Bente. 1999. Documentation of active peregrine falcon nest sites, 1 Oct 1994- 31 March 1998. Alaska Department of Fish and Game. Annual research report. Endangered species conservation fund federal aid project SE-2-9, 10, and 11. Juneau, AK. 15 pp.
[4] Cape May Bird Observatory. Cape May Hawkwatch, Cape May, New Jersey. New Jersey Audubon Society. Website (http://www.njaudubon.org/Sightings/cmhw25.html) accessed October 31, 2005.
[5] NatureServe. 2005. NatureServe’s Central Databases. Arlington, VA. U.S.A.