What prevents birds from interbreeding with other species?

Put simply, for many birds, song acts as a ‘password’: if a potential mate sounds too different, the likelihood of inter-species mating is lower. This helps species maintain their boundaries, even when closely related species live nearby.

This is the conclusion reached by researchers at the Estación Biológica de Doñana — a Spanish institute that forms part of the CSIC.

Their work has been published in the journal *Biology Letters*. The authors compared data on bird hybridisation with differences in their songs and plumage colouration.

Details

Hybridisation is a situation where individuals of different but closely related species interbreed. This results in the production of hybrids. In nature, this occurs quite frequently, especially if species inhabit the same territory and may encounter one another during the breeding season.

However, species usually have barriers that prevent them from interbreeding. One such barrier is pre-mating behaviour: a bird must recognise ‘its’ mate. Signals such as voice, plumage colour, movements and courtship displays are important for this.

In a new study, scientists investigated which factor is more strongly linked to the restriction of hybridisation: song or plumage colour. They used data on recorded cases of hybridisation between bird species and then compared these with quantitative assessments of differences in acoustic signals and plumage colouration. The analysis also took into account the relatedness of the species and whether their ranges overlapped – in other words, whether they could actually encounter one another in the wild.

The result was quite clear: species with more distinct songs hybridised less frequently. Even when such birds lived in the same region, marked differences in vocal signals were associated with fewer instances of hybridisation.

The picture was less clear when it came to plumage colour. Differences in the colouration of males did not predict the likelihood of hybridisation as well as songs did. In females, the link was more pronounced: if the plumage of females in closely related species differed significantly, hybridisation also occurred less frequently. However, acoustic signals still proved to be the main factor.

A bird often hears a vocal signal before it sees a potential mate. In dense vegetation, at a distance or in a complex environment, a song may be the first indication of who is nearby.

Furthermore, song is well suited to species recognition. If two closely related species look similar but sing differently, a bird can avoid a mistake even before approaching. This reduces the likelihood of inter-species mating.

Feather colour is also important, particularly in mate selection and courtship behaviour. However, it works differently: the bird must be seen, and plumage colour can vary depending on lighting, age, sex, season or environmental conditions. Therefore, across many species, song may have proved to be a more reliable ‘species-specific signal’.

The authors of the study believe that their research has, for the first time, demonstrated a link between acoustic differences and hybridisation in birds at such a broad evolutionary level. Previously, the role of song was more often studied at the level of individual species or families.

What this means in simple terms

If two closely related bird species live nearby, they could potentially interbreed. But for this to happen, they need to ‘accept’ one another as suitable mates.

Song helps to avoid such a mistake. The more the voices of two closely related species differ, the less likely the birds are to perceive each other as a mate.

Therefore, a bird’s song is not just a beautiful sound. It is an important biological signal that helps birds find their own kind, avoid unsuitable mates and maintain differences between species.

Why this is important

This research helps us understand how species boundaries are maintained in nature. This is one of the key questions in evolution: why closely related species do not always interbreed, even if they live side by side and are capable of producing hybrid offspring.

Such barriers are important for the emergence of new species. If two populations gradually begin to sing differently, they may interbreed less and less. Over time, this can reinforce the division between them.

This research is also important for understanding the consequences of environmental change. If species’ ranges begin to overlap due to climate change, habitat destruction or human activity, different species may encounter one another more frequently. In such circumstances, it is particularly important to understand which signals help prevent them from interbreeding.

However, the conclusion should not be oversimplified to the notion that ‘birds choose a mate based solely on song’. Mate selection is more complex: voice, appearance, behaviour, habitat and the opportunity to meet are all important factors. New research shows precisely that, across a large sample, differences in song were found to be more strongly associated with the restriction of hybridisation than plumage colouration.

Background

Scientists have long known that birds use several signals simultaneously when choosing a mate. Males of many species sing, display their plumage, stake out a territory or perform courtship displays. Both males and females may evaluate these traits to select a mate of their own species and of suitable quality.

Hybrids among birds have also been known for a long time. For example, the Italian sparrow is often regarded as a stable hybrid between the house sparrow and the Spanish sparrow. There are other groups where hybridisation occurs regularly, particularly where species are genetically closely related and their ranges overlap.

But the question remained: what better protects species from interbreeding — differences in song or differences in appearance? New research shows that, in a broad comparison, vocal signals may be a more important barrier.

This does not negate the role of plumage. In some species, feather colour can be critically important. But when looking at birds as a whole, song proved to be a stronger indicator of whether closely related species would interbreed.

Source

Study: Vicente García-Navas, Joaquín Ortego, “Song but not colour divergence constrains hybridisation in birds”, Biology Letters, 2026.