Urbanization promotes hybridization between Common Swift and Pallid Swift

Genetic analyses uncover several hybrids and backcrosses in France.

In 2013, French researchers discovered a mixed pair of Common Swift (Apus apus) and Pallid Swift (Apus pallidus). This observation raised the question how common hybridization is between these two species. Identifying hybrids based on morphological traits is extremely challenging. Indeed, even “pure” Common and Pallid Swifts are difficult to tell apart. That is why Alice Cibois and her colleagues turned to genetic data. They inspected the genetic make-up of almost 500 individuals.

The sampling effort centered around the French town of Bastia (Corsica) where both species are breeding. The researchers noted that “although the two species are known to form mixed colonies at the same natural sites, sympatry predominantly occurs within urban regions where both species breed in buildings.” The chances of finding hybrids are thus highest in these urban areas.

Recent Hybrids and Backcrosses

The mitochondrial gene COI provided the first clue for hybridization. A haplotype network uncovered six Pallid Swift individuals with mitochondrial sequences of the Common Swift. Nuclear markers – a set of nine microsatellites – provided more detailed patterns of hybridization. The analyses suggested four first-generation hybrids and ten backcrosses (indicating that the hybrids are fertile). These findings highlight the power of genetic data to document hybridization between morphologically similar species. The researchers nicely address this topic in the discussion of their paper:

Observers never reported a mixture of phenotypic traits suggestive of a hybrid origin. Genotyping at nuclear markers is thus the only tool available to reliably identify individuals with hybrid origin and to track the dynamics of introgression between the two species.

Haplotype network for Common Swifts (blue) and Pallid Swifts (orange). The circles are proportional to the number of individuals. Asterisks show the six individuals identified as Pallid Swifts that have Common Swift haplotypes. From: Cibois et al. (2022).

Anthropogenic Hybridization

As expected, most of these admixed individuals were found in and around the city of Bastia. This pattern supports the idea that human actions can lead to hybridization. And it is not just limited to swifts. In a previous blog post, for example, I explained how nectar feeders and ornamental plants in gardens might have facilitated hybridization between two subspecies of Allen’s Hummingbird (Selasphorus sasin). Similarly, another recent study reported that hybrids between four species of chickadee (genus Poecile) were more common in urban settings (see this blog post). Urbanization might thus fuel hybridization.


Cibois, A., Beaud, M., Foletti, F., Gory, G., Jacob, G., Legrand, N., … & Thibault, J. C. (2022). Cryptic hybridization between Common (Apus apus) and Pallid (A. pallidus) Swifts. Ibis164(4), 981-997.

Featured image: Common Swift (Apus apus) © XJochemx.nl | Wikimedia Commons

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