Song birds that have been artificially introduced to isolated areas are fruitful material for investigating changeability of songs within a limited period of time. I studied songs of Japanese Bush-warblers, Cettia diphone, which were introduced from Japan to the island of O‘ahu (Hawaiian Islands) ca. 80 yr ago. These warblers on O‘ahu sang acoustically simpler songs at lower frequencies than the warblers on Honshu, the main island of Japan. Previous studies found similar tendencies on small peripheral Japanese islands. Morphological characteristics indicated that the warblers on the Hawaiian Islands did not originate from insular subspecies in Japan. Therefore, the acoustic structure of their songs may have changed during their 80 yr on O‘ahu. Possible factors driving this rapid change are relaxed sexual selection and/or the sound transmission properties of the island habitat.