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Mitochondrial DNA Reveals a More Complex Evolutionary History of Barn Swallows

First-year barn swallow perched on a thin branch against a green background
Research · Genetics & genomics

A large genetic study published in August 2026 shows that the evolutionary history of the barn swallow (Hirundo rustica) is more complex than earlier mitochondrial studies suggested. The researchers analysed mitochondrial DNA, a small genome found in the cell’s energy-producing mitochondria and normally inherited through the maternal line.

The study included 580 complete barn swallow mitochondrial genomes representing all six recognised subspecies, together with mitochondrial genomes from 12 other species in the genus Hirundo. The researchers identified 553 different haplotypes, or distinct mitochondrial DNA variants. Of these, 166 had not been described before.

Five maternal lineages included a newly recognised haplogroup

The barn swallows fell into five major mitochondrial haplogroups. A haplogroup is a set of related maternal lineages that share a common evolutionary origin. One of the main findings was a newly recognised haplogroup, called E, found mainly in H. r. tytleri. Earlier mitochondrial work had placed this lineage within the group associated with the American subspecies H. r. erythrogaster.

Maps showing geographic frequencies of five main barn swallow mitochondrial haplogroups
Geographic frequency distribution of the five main mitochondrial haplogroups identified in barn swallows. Lombardo et al. (2026), CC BY 4.0.

Comparisons with other Hirundo swallows also support a southern African origin for the lineage leading to modern barn swallows toward the end of the Pliocene, followed by later expansion and diversification across Eurasia and the Americas. Many of the major maternal lineages appear to have diversified during the Late Pleistocene, when repeated climatic changes altered habitats and migration routes.

Hybrid zones showed maternal introgression between subspecies

The data also reveal extensive contact between subspecies. In several hybrid zones, birds identified as one subspecies carried mitochondrial DNA typical of another. This is evidence of maternal introgression: genetic material has moved between populations through hybridisation followed by repeated breeding back into one of the parental populations.

Mitochondrial DNA traces only the maternal part of the species’ history, so it cannot by itself resolve every relationship between the subspecies. Nuclear genomes, inherited from both parents, are needed to test some of the remaining questions. Even so, the 580 complete mitochondrial genomes form the most detailed mitochondrial picture of barn swallow population history produced so far.

Sources and image credit

  1. Lombardo, G., Gallo, G. R., De Benedictis, A., Cavallini, M., Formenti, G., & Binelli, G. (2026). Mitogenomes reveal novel phylogeographic patterns and temporal evolutionary history of barn swallows and close relatives. International Journal of Molecular Sciences, 27(16), 7433. https://doi.org/10.3390/ijms27167433Primary peer-reviewed open-access study, published 20 August 2026; CC BY 4.0.

Image: TRinaud — first-year barn swallow photographed in Zichow, Brandenburg, Germany; representative image. Source: Wikimedia Commons. · CC BY 4.0 · Source

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