Birdsong plays greater role than plumage in preventing hybridisation
A large-scale study has found that closely related bird species with distinctive songs are less likely to hybridise, while differences in male plumage appear to provide little protection against interbreeding.
Differences in birdsong may be considerably more important than colourful male plumage in maintaining boundaries between closely related bird species, according to new research.
Scientists at the Estación Biológica de Doñana in Spain compared recorded cases of avian hybridisation with measurements of song and plumage divergence. Their analysis covered 657 species from 84 genera and 16 families, with the dataset dominated by oscine passerines.
Species whose vocalisations differed most strongly from those of other members of their genus had significantly fewer recorded hybridisation partners. After geographical overlap and other factors were taken into account, a one-standard-deviation increase in acoustic divergence was associated with an estimated 24% reduction in hybridisation frequency.
The researchers found no equivalent relationship for male plumage. Species with males that looked markedly different from their closest relatives were no less likely to hybridise than those with comparatively similar male plumages.
Female coloration produced a different result, with greater divergence associated with a small reduction in hybridisation. However, this effect was weaker and less consistent than that recorded for song.
Song is already recognised as an important reproductive barrier in many passerines. It can be detected over long distances, allowing birds to identify potential mates before visual signals can be assessed at close range. Learned songs may also change relatively quickly through cultural transmission, enabling populations to develop distinctive vocalisations faster than differences in plumage evolve.
The findings may help explain why some groups contain visually similar species that are most reliably separated by voice. In Scytalopus tapaculos, for example, plumage variation within a species can be greater than that between species, and vocal differences are consequently central to both identification and decisions over species limits.
Conversely, genomic studies have shown that hybridisation and gene flow can continue between birds with strikingly different male plumages. Genes controlling conspicuous ornamental traits may cross species boundaries while much of the remainder of the genome remains less affected, weakening the relationship between visual distinctiveness and reproductive isolation.
The researchers accounted for the degree to which each species overlapped geographically with related species. As expected, birds with more sympatric congeners had a greater opportunity to hybridise, but the negative relationship between song divergence and hybridisation remained strong after this was considered.
The study was based on documented hybrid records and existing global datasets of acoustic and plumage characteristics. The authors noted that hybridisation is more likely to be recorded in well-studied groups, while their plumage measurements did not include ultraviolet reflectance or fully represent the way birds themselves perceive colour.
The analysis was also correlational and could not determine conclusively whether distinctive songs prevent hybridisation or whether previous hybridisation has contributed to greater similarity between the songs of related species.
The results nevertheless provide broad comparative evidence that acoustic signals form a major pre-mating barrier across birds, while the contribution of plumage to species recognition appears to be more dependent on sex and evolutionary context.
July 2026
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