Members of the genus Zosterops are known for their colonizing ability and extensive phenotypic differentiation on numerous islands. There have been morphological and biochemical analyses of some Zosterops populations, but little study has been devoted to patterns of vocal communication signals, known to be important pre-mating barriers in many bird species and in possible diversification of taxa. I report on the song system of one subspecies of Zosterops in a mainland population and an island population 15 km distant. I used both a traditional subjective classification of song elements and the multivariate procedure of linear discriminant analyses (LDA) of measured sound features. The syllables constituting songs exhibited a low level of stereotypy, disallowing a lexicon of syllable ‘types’ to be constructed for individuals or a population. New syllables were continuously produced as a bird uttered more and more songs, possibly indicating an extremely large repertoire or an open-ended generation of vocal innovations. LDA indicated songs of the island population were moderately differentiated from and less variable than those of the mainland. This type of song system creates a problem for research on vocal signals, whether directed at comparisons between birds in a local area or between populations. I made a preliminary effort to address this problem and discuss my results in the framework of Zosterops and its propensity for evolutionary diversification.