Habitat-dependent song variation could play a role in ecological speciation from the incipient stages until completion of reproductive isolation. The evolutionarily novel urban habitat provides an excellent opportunity to study this role for song in the initial steps of population divergence associated with contrasting habitats close to home. In this study, we show spectral and temporal song divergence at population level in European blackbirds in 3 city-forest pairs. The observed upward spectral shift in frequency for urban songs relative to forest songs is consistent with previous studies on the same and several other species. A reciprocal playback experiment showed that male blackbirds pay attention to the origin of the stimulus and the divergent spectral trait independently of each other, as well as to a combination of both. The fact that urban and forest males responded differentially to the same spectral song features suggests that the song divergence could influence where males with typical urban or forest songs settle and are able to maintain a territory. Our findings show that habitat-dependent song divergence has the potential to affect the responsiveness to signals in the initial stages of population differentiation.
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