Relationships among song characteristics in Japanese tits

Birdsongs often contain two simultaneous, harmonically unrelated sounds; however, little is known of the function of this two-voice phenomenon. We hypothesized that it contributes to increases in song speed and note repertoire, which are important song characteristics in sexual selection. We analyzed six song variables in Japanese tits (Parus minor) and found positive correlations between all possible pairs of the proportion in length of two-voice vocalization, note rate, and note type repertoire, which is consistent with our hypothesis. This finding is a first step towards determining the function of two-voice vocalization in song communication.