Simplified song in an island population of the bush warbler Cettia diphone

Song complexity and the repertoire of the bush warbler Cettia diphone were studied in an island and a mainland population in Japan. The song complexity (number of modulations in a song) was lower in the island population than in the mainland one. On the other hand, the repertoire size (number of song types per male) was larger on the island. Founder effect probably does not influence the island song because colonization occurred a very long time ago. Sound transmission properties of the habitat and weak selection pressure for interspecific identification might have influenced the island song, but these cannot elucidate the simpler song on the island. There is a possibility that social conditions affect song complexity in the populations; the highly polygynous mating system of the mainland population yields strong selection pressure toward complex song through acquisition of mates and/or establishment of quality territories. On the other hand, variable songs on the island seem to be affected by cultural mutation. Received: September 3, 1999 / Accepted: December 9, 1999