In such cases, the need for individual recognition and distinctiveness may favour individuals that produce innovative or unusual songs 24, 25, thereby eliminating local dialects 24, 25, 26. However, conformity may be limited when sexual selection favours greater song complexity for individual males 21, 22 or when benefits of signalling individual identity 23 favour greater variability between males. Cultural conformity within local dialects ensures that the signal will be recognised by receivers 18. Just like the diversification of human languages and dialects 15, 16, geographically separated populations of animals with learnt vocalisations (mostly passerine birds) have diverged culturally into geographically restricted song dialects 3, 4, 17, 18, 19, 20. Such culturally inherited traits may be passed on from one generation to the next with imperfect fidelity, leading to divergence between isolated populations via cultural drift 10, 11, 12, 13, 14. From the receiver side, recognition of song is also learnt, typically involving sexual imprinting either on parents or on other members of the population 6, 7, 8, 9. In many species, including in primates 1, cetaceans 2 and birds 3, 4, individuals learn song or contact vocalisations from social interactions with their parents or with other conspecifics 5, 6. Our study shows evidence that in zebra finches, a model species for song learning, individuals are sensitive to differences in song that have hitherto remained unnoticed by researchers. We find that females preferentially pair with males whose song resembles that of the females’ adolescent peers. We cross-fostered eggs within and between these populations and used an automated barcode tracking system to quantify social interactions. We examine mating patterns across three consecutive generations using captive populations that have evolved in isolation for about 100 generations. Here we show that machine learning can nevertheless distinguish the songs from multiple captive zebra finch populations with remarkable precision, and that ‘cryptic song dialects’ predict strong assortative mating in this species. However, the emergence of song dialects has been considered unlikely when songs are highly individual-specific, as in the zebra finch ( Taeniopygia guttata). Culturally transmitted communication signals – such as human language or bird song – can change over time through cultural drift, and the resulting dialects may consequently enhance the separation of populations.
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