Song Divergence in Hummingbird and Junco Populations on Guadalupe Island

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Abstract
Both the anna hummingbird [Calypte anna] and the junco [Junco hyemalis] population on Guadalupe Island [in the Pacific Ocean] differ in song from mainland populations. Using playback experiments, differences between the songs of island and mainland males of C. anna were shown to evoke different overt responses in mainland males; females showed no overt response to either island or mainland male songs. Male Guadalupe Island C. anna showed no apparent difference in color or external morphology from mainland populations. The song of the insular junco population is both different from and highly variable compared to the song of mainland J. hyemalis. The Guadalupe Island junco also differs from related continental populations in coloration and morphology. Such differences presumably indicate considerable genetic differences between mainland and island populations which could result in reproductive isolation and specific distinctness. The observed divergence in song of both C. anna and J. hyemalis on Guadalupe Island may have arisen from lack of opportunity in a founder population for learning the song of mainland adult males. A further explanation, particularly applicable to J. hyemalis, is that in an environment with a small number of bird species present, selection for distinctiveness in song may be reduced, leading to increased variability and ultimately to song divergence.

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