The populations of Peregrine Falcons underwent significant fluctuations in the 20th century. In places where its numbers had fallen, the great ability of Peregrines to adapt led to a recovery in its populations. However, in many areas numbers have yet to recover from pre-fall levels.
The first great population decline of the Peregrine Falcon in the 20th century took place in Great Britain during the Second World War (1939-45), when Peregrines were victims of an extermination campaign on the part of the Air Ministry. The aim was to reduce the number of messenger pigeon losses, then of great value for sending military information and mail, killed by the Peregrines. The campaign was begun on 1st July 1940 and ended in February 1946. During this period more than 600 individual birds were eliminated and it is calculated that the British nesting population fell by 83%.
Nevertheless, the most substantial impact at a world level -the reason for which the populations of the species fell so significantly, even leading to its extinction in some places in the planet- can be found in the deadly effect of organochloride pesticides used massively in agriculture. Compounds which contained DDT and their most important metabolite, DDE (1,1-dichloro-2,2-bis(p-chlorophenyl)ethane and cyclodienes (aldrin, dieldrin and heptachlor) and polychlorobiphenyl (PCBs) are highly stable and soluble in fats. This means that they pass easily from prey to predator and that they can accumulate in the adipose tissues of animals, concentrating primarily in the highest points of the food chain, which is precisely where the Peregrines are positioned. This phenomenon is known as bioaccumulation.
Dead Peregrine
These substances were widely used as insecticides to combat crop pests. The effect of these substances began to appear at the end of the 1940s, but it was during the fifties and sixties when the effects became more pronounced with a significant fall in the populations of Peregrine Falcons in most countries in the Northern Hemisphere. During the 1970s the bird disappeared completely from the eastern half of the United States and adjacent areas of Southern Canada. This also took place in many European countries. The populations of Peregrine Falcons completely disappeared from Belgium, Luxemburg and the former German Democratic Republic.
Herbicides
The populations which did not disappear suffered from sharp declines. Austria, for example, saw its population fall by 86%, the former Federal Republic of Germany by 98%, Great Britain by 66%, and in Norway the population dropped from an estimated 500-1000 pairs at the start of the century to just 10 pairs in 1975.
A range of organochloride pesticides causing diverse forms of alteration have been blamed for negatively affecting reproduction in birds, as they bioaccumulate in body fat and become mobilised during the breeding season. The most frequent alteration is the interference in the calcification of egg shells and the reduction in their thickness, frequently resulting in the female -when incubating- breaking the eggs because of the shell's fragility. For example, 15-20 ppm of DDE in fresh weight of the animal results in a reduction of 17% in the thickness of the shell. Other alterations described include an inability to lay eggs, an increase in unfertile eggs, the abandonment of the clutch and the death of embryos.
Once the use of DDT and other organochloride pesticides was prohibited, there was a slow recovery in the populations of Peregrines. In some cases this recovery was the result of the work done by organisations created with this purpose in mind. Among the most important are the Peregrine Fund in the USA and the Canadian
Peregrine Foundation in Canada. Despite all the efforts made, there are still today significant levels of these toxins in eggs and therefore shell thinning is still recorded. The populations of Peregrines which migrate are in greater risk of contamination with organochlorides because DDT is still used, illegally, particularly in Third World countries. It should also be taken into account that Peregrines which do not migrate (sedentary birds) are exposed to contaminants when they feed on migratory prey.
All in all, the world population of the Peregrine Falcon is currently estimated at a minimum of 20,000 pairs. In Europe, the population is estimated at some 7,600-11,000 pairs and in Catalonia the population is stable with 245-253 pairs but with a slightly upward trend.
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