Abstract
Geographical variation in the structure of mating signals can influence the processes of mate recognition and reproductive isolation. In Darwin's finches of the Galapagos Islands, Ecuador, prior studies of geographical variation in vocal mating signals (songs) have focused on populations on small islands, on which there is presumably little opportunity for the evolution of within-island song variation or discrimination abilities. The situation on larger islands, however, is less clear. I characterized aspects of song variation and discrimination in the medium ground finch, Geospiza fortis, in the southeastern coastal region of the large island of Santa Cruz. Songs from two sites, Bahia Academia and El Garrapatero, separated by approximately 11 km, differed in mean minimum frequency and in a corresponding principal component factor, and could be distinguished with moderate success in discriminant function analysis. Territorial males at El Garrapatero showed strong discrimination of songs by site: in response to playback of local songs, birds showed higher rates of singing and flying, and significantly closer approaches to the playback speaker. Successful discrimination of songs by site, in the face of overlap in song structure between these sites, suggests that these birds rely on an exemplar-based mechanism of song perception. Moreover, the results support the hypothesis that birds in these populations are relatively philopatric, and thus suggest an opportunity for within-island evolutionary divergence.
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