Singing in the sky: song variation in an endemic bird on the sky islands of southern India

Birdsong structure is known to vary across different scales of geographical separation, from differences between neighbours in a habitat to populations across continents. The high-elevation regions of the Western Ghats mountains in southern India form ‘sky islands’ containing the unique Shola habitat. Bird species on such sky islands are often specifically adapted to habitats typical of these islands while populations on different islands may have been geographically isolated over varying periods of time. Forest fragmentation can intensify the effects of such isolation by affecting species dispersal processes. We examined the effects of genetic differentiation across populations on the song of a threatened, endemic bird, the white-bellied shortwing, Brachypteryx major, on different islands of this sky island system. We compared songs from three populations, one of which on one island was genetically distinct from the other two populations on another island. These two populations were genetically similar but separated by recent deforestation. We recorded songs from 23 individuals and characterized 572 songs by 13 parameters. Multivariate analyses revealed significant differences in song between the three populations, with the genetically distinct populations across the two islands being the most differentiated. This was supported by a visual and aural examination of spectrograms that revealed characteristic qualitative differences in songs across these populations. Finally, this study corroborates accepted patterns of congruence between song and genetic divergence across islands and also highlights the difference in song between anthropogenically fragmented, but genetically similar populations, possibly owing to cultural drift.