Variation in the song of the aquatic warbler Acrocephalus paludicola in response to playback of different song structures

Songs of the aquatic warbler (Acrocephalus paludicola) can be classified on a structural basis into three main types, A, B and C. A songs are the shortest, and consist of one, simple phrase. B songs consist of the same simple phrase, followed by a second more complex phrase. C songs are the longest and most complex, consisting of more than two such phrases. During spontaneous singing, males produced approximately equal numbers of B and C songs. During playback, males produced hardly any C songs, about the same number of B songs, and large numbers of A songs. The number of A songs produced was inversely correlated with distance of approach to the speaker. When a mixture of A and B songs were played, males produced more A songs in response but did not approach so near to the speaker. When only C songs were played, males produced fewer A songs but approached closer to the speaker. The evidence suggest that A songs are only used as a short range aggressive signal, and B songs mainly as a long range territorial proclamation. Although there was no direct evidence, C songs may function as a long range sexual attractant. When studies on other Acrocephalus species are also considered, the overall trend is for intrasexual selection to produce short, simple territorial songs and intersexual selection to produce longer, more complex songs for mate attraction.