Male song of the Aquatic Warbler, a promiscuous bird without paternal care, is more complex than previously thought.

The Aquatic Warbler Acrocephalus paludicola is one of the rarest European passerines and is characterised by promiscuity, lack of pair-bonds and female-only parental care. This makes the species an important model for studying the function of avian courtship song. The song of the Aquatic Warbler consists of whistle and rattle phrases produced as discontinuous A-, B- and C-songs, which are built by a single rattle, a rattle and a whistle, and more than two phrases of both kinds, respectively. The A- and B-songs are thought to be aggressive signals in male-male interactions, while C-songs are thought to be important for female choice. Here, we analysed recordings of 40 individually marked males, and determined the phrase repertoire. The enumerated repertoire (males recorded for ≥ 10 min) ranged from 16 to 158 (mean 99), however, it did not capture the complete phrase repertoires. We then used models from species diversity ecology to estimate the actual phrase repertoire size, which ranged between 18 and 300 phrases (mean 155). The estimated repertoire was predicted by the number of C-songs. The rattle repertoire was larger than the whistle repertoire, and both positively correlated with the number of C-songs. Our study indicates that male Aquatic Warblers have highly complex phrase repertoires that vary widely in size. Their courtship song is flexible and efficient, enabling relative song complexity to be demonstrated in a short sample, thus facilitating both female attraction through the quick presentation of large phrase repertoires and rival deterrence through the production of many short and simple A- and B-songs.