Contrasting patterns of geographical song variation in two closely related passerine species with a simple song

Decades of research on geographic variation of birdsong have provided evidence that passerine vocalization often diverges among populations. We asked whether even songs so simple that they superficially resemble stridulating insects vary geographically. We focused on two closely related species of the genus Locustella, the River Warbler (L. fluviatilis) and the Grasshopper Warbler (L. naevia). At four Central European localities separated by 85–380 km, we recorded 62 River Warbler males, and at three of these sites, we also recorded 32 coexisting Grasshopper Warbler males. We hypothesized to observe differences among geographically distant populations in both species. However, only the song of River Warbler diverged among the localities in structural and quantitative parameters, especially in the number, frequency and position of high-amplitude notes within the repeated syllables. Discriminant analysis successfully classified 80% of all River Warbler males to their respective population, in agreement with our subjective classification of songs into several categories. In contrast, the populations of Grasshopper Warbler at the same spatial scale could not be differentiated either by visual inspection or by any of the measured song characteristics. Further comparison with spectrograms available from the European range of both species supported these patterns also on a larger geographical scale, with additional distinct River Warbler song types observed out of our study region, but similarly looking Grasshopper Warbler song types distributed across the continent. Different patterns of song geographic variation in the two coexisting, closely related species highlight species-specific traits that contribute to song divergence and imply the great diversity in singing behaviour among songbirds.