Geographic variation in type I dialects of Hermit Warblers: does fragmented habitat promote variation in song?

Abstract

We evaluated the role of habitat fragmentation in promoting song variation and the potential for the generation of dialects within four type I dialect areas of Hermit Warblers (Setophaga occidentalis). The dialect areas differed markedly in the degree of habitat fragmentation due to topographic relief, aspect, and elevation. Geographic variation in song was greatest in the most highly fragmented area, but substantial variation existed in all dialect areas including the area with no obvious fragmentation. Songs in isolated and partially isolated portions of a dialect area differed from songs in the more continuous areas, but geographic variation was also found within the continuous portions of dialect areas. Much of the variation in song structure was clinal, crossing discontinuities in habitat. Within a dialect area, songs varied geographically with differences often apparent over distances ≤10 km in continuous habitat. Despite the great geographic variation in song, evidence that fragmented habitat promotes song variation is insufficient to explain the observed pattern of variation in type I song. Noticeable song variation over relatively small distances suggests males converge on similar type I songs locally while periodic large-scale disturbances, primarily wildfire, are likely important in the partitioning of variable dialects sufficiently in space and time for the generation of new dialects.