How did the population of Sussex Peregrine Falcons recover?

Searching for the source population of these UK birds.

The history of the Peregrine Falcon (Falco peregrinus) is a string of ups and downs. This enigmatic falcon species has been prosecuted numerous times and suffered from pollution, only to bounce back afterwards. During the Second World War, Peregrine Falcons were shot because they predated on homing pigeons, which carried important military messages. After the World War, they were targeted by grouse-moor gamekeepers (i.e. the hunting of Red Grouse, a field sport in the UK) and pigeon fanciers. In addition, the use of pesticides – specifically DDT – posed another threat. These chemicals flow through the food chain, eventually accumulating to high doses in predators where they lead to the thinning of eggshells and a reduction in breeding success.

Luckily, the use of these chemicals has been banned and the prosecution of Peregrine Falcons has diminished. In several regions, such as Scandinavia and North America, re-introduction programs have restored the breeding populations of Peregrine Falcons. In the southern part of the UK, however, the number of Peregrine Falcons seems to have recovered without any human assistance. In Sussex, the number of breeding pairs increased from ca. 10 in 1954 to more than 40 by 2016. This remarkable population growth raises an important question: where did these birds come from?

Source Populations

In a recent Conservation Genetics study, Angela Weaving and her colleagues attempted to determine the origin of the Sussex Peregrine Falcons. Using a combination of microsatellites and mitochondrial markers, they tested several hypotheses about the identity of the founding birds:

  • Migrants from the UK
  • Migrants from mainland Europe
  • Escaped captive birds (possibly including hybrids)

To discriminate between these possible source populations, the researchers compared the genetic material of the Sussex birds with a historical sample (before the 1950s) of Peregrine Falcons from the UK, domestic-bred birds from the UK, and wild birds from Germany, the Republic of Ireland and the Mediterranean.


Analyses of the mitochondrial DNA were not very helpful to solve this mystery. One main haplotype was shared by all the sampled populations. This low genetic diversity is probably a remnant from a rapid population expansion from a small population (with limited genetic diversity) at the end of the Pleistocene. The microsatellites, on the other hand, did provide insights into the origin of the Sussex population. The researchers found that “the contemporary wild Peregrine population in Sussex is genetically similar, but not identical, to the pre-pesticide UK population, and it is genetically different from other European populations and from the domestic stock.” It thus seems that the Sussex population increased through the influx of other Peregrine Falcons from the UK. More sampling is needed to pinpoint the exact locations where these birds came from.

The contemporary population (dark-blue) clusters with the UK population from the pre-pesticide period (light-blue). There is a clear difference with the domestic birds (orange). From: Weaving et al. (2021) Conservation Genetics.

Hybrid Falcons

Hybrid falcons, such as Peregrine Falcon x Gyrfalcon (Falco rusticolus), are quite common in captivity and regularly escape into the wild. Vincent Fleming and his colleagues estimated that since 1980 at least 30 Peregrine Falcon hybrids per year have been reported as lost. This is probably an underestimate because it is now no longer required to report escapees. The findings from the Sussex study suggest that escaped hybrids did not markedly influence the genetic make-up of the wild population. It could be that these hybrids might have lower survival rates or difficulties in finding a partner. However, these birds could still affect the wild population by occupying breeding territories or engage in failed nesting attempts with “pure” Peregrine Falcons. More research is thus needed to understand the fate of escaped hybrids, and their impact on wild populations of Peregrine Falcons.

The number of falcon individuals that were reported as lost in the UK. From: Fleming et al. (2011).


Weaving, A., Jackson, H. A., Nicholls, M. K., Franklin, J., & Vega, R. (2021). Conservation genetics of regionally extinct peregrine falcons (Falco peregrinus) and unassisted recovery without genetic bottleneck in southern England. Conservation Genetics22(1), 133-150.

Featured image: Peregrine Falcon (Falco peregrinus) © Mosharaf Hossain Ce | Wikimedia Commons

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