Asymmetric behavioural responses to divergent vocal signals in allopatric Neotropical sparrows

Birdsongs are signals that encode information for multiple behaviours such as species recognition. Species recognition is influenced by both geographical variation in song and birds' discrimination abilities, forming the basis for evolutionary processes driving reproductive isolation and speciation. Here, we assessed the behavioural responses to geographical variation in songs in olive sparrows, Arremonops rufivirgatus. We conducted a series of playback experiments in two allopatric populations (Pacific, Peninsula) to assess whether olive sparrows responded differentially to geographical variants of their song and whether such responses could be predicted by the acoustic similarity between subject and stimulus songs. Our results indicate an asymmetrical behavioural response, with the Pacific population responding strongly to both local and allopatric stimuli and weakly to the control, and the Peninsula population responding strongly to local stimuli and weakly to both allopatric stimuli and the control. Furthermore, despite the birds' lack of previous experience with the playback stimulus songs, we found that response intensity was predicted by the acoustic similarity in both populations, with stronger responses to more similar songs, suggesting that males from different population may use a similar mechanism to recognize signals, despite whether the signal comes from conspecifics or heterospecifics. Our findings support the hypothesis that song divergence could act as a premating barrier for at least one of the studies populations, and that birds' responses are dictated by the structural similarity between senders' and receivers’ signals.