Singing of Claudia’s Leaf-warbler (Phylloscopus claudiae) in aggressive contexts: role of song rate, song type diversity and song type transitional pattern

In many songbirds, males vary certain aspects of their singing behavior when engaged in territorial interactions. Song rate, song type switching rate, song matching, song overlapping, and the usage of specific song types were proposed to be aggressive signals. It is well known that transitions between different song types in a song sequence are non-random in many species, but the signaling significance of non-random vocal streams is poorly understood. We asked whether singing plays a role in male–male interactions in Claudia’s Leaf-warbler. Along with more traditional metrics (i.e., song length, song rate, song type switching rate and song type diversity), we used (1) song sequence linearity (SLIN) and consistency (SCONS) indexes, and (2) applied an information theory approach by means of a first-order relative entropy (RE1) calculation to analyze the role of song type transitional patterns in male–male interactions. The study was conducted in April–June 2016 in Hupingshan National Nature Reserve, Hunan province, China. We simulated territorial intrusion by broadcasting Claudia’s Leaf-warbler songs in territories. Experiments involved 14 different males. A comparison of spontaneous singing with that elicited by playback showed that song rate and song type diversity increased, and that entropy decreased. By contrast, SLIN and SCONS, though significantly correlated with RE1, did not differ between spontaneous singing and singing in response to playback. The decrease of entropy means that the transitions between different song types in a song sequence were determined more by specific factors and predictable and thus more non-random. However, these results were weak, as the decrease of RE1 did not coincide with the expected increase of SLIN, SCONS and S. Our results are thus partly consistent with the idea that non-random vocal structures, along with other song parameters, could play a role in male–male competition.