A three-part study of the vocal and visual cues important for species recognition in six species of Darwin's Finches (Geospiza) in the Galapagos is presented. Part I quantifies structure and species-specificity of advertising song. Part II describes experiments on song function and recognition. Part III examines the role of morphology in mate recognition and reproductive isolation. Geospiza song is variable and lacks species-specificity, due to intrapopulation song polymorphism, dialect divergence, song parallelism and interspecific song overlap. Song functions in conspecific territorial communication. Much intraspecific song variation apparently has little functional significance, although vocal confusion between syntopic G. fuliginosa and G. fortis may exist. Visual recognition involves morphological (non-plumage) stimuli of both head and body. Male recognition of potential mates functions in reproductive isolation, and character displacement in recognition ability occurs. Visual cues appear to override vocal ones in short-range communication. Lack's (1945) hypothesis that Geospiza use characters specialized for feeding as mate identification cues is confirmed. Low interspecific song divergence is probably related to the evolution of visual species recognition.

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