We recorded male songs in a natural forest island (isolated for 3000 years), and at two nearby sites in continuous forest. A previous study found the isolated population to be weakly differentiated genetically from the ancestral population, suggesting that peripatric speciation was underway. We detected minor but significant differences in song structure between each site, with the most divergent songs being those of island birds. On simulating secondary contact using playback, we found that pairs from the forest island responded more strongly to island (i.e. local) songs than to those from both non-island sites, and vice versa. Island females were more likely to approach and sing after hearing local male songs, rather than songs from the non-island populations, and vice versa. Meanwhile, non-island females did not appear to discriminate between local songs and those from the other non-island site.

These findings are consistent with the idea that vocal divergence arising in small populations may result in partial reproductive isolation when contact is resumed. They also suggest that song divergence in peripatry may, after much longer time-frames, act as a barrier to gene flow in suboscines, perhaps because of an inability to learn or recognize divergent songs on secondary contact. The take-home message is that high species richness in Neotropical suboscine birds might have been partly driven by peripatric speciation events at the edge of Amazonia, and elsewhere.

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