<p>Songbirds are well known for their versatile singing. Increased song versatility (number of different song or syllable types per a given time interval), which could be realized by increasing song diversity and/or switching rate, is known to play a role in male-male interaction. However, the evidence is still mixed. We investigated male–male communication in black-faced buntings. Males of this species lack stereotyped song types. Instead, each male possesses a repertoire of syllable types that are combined into songs. All songs of a male could be divided into classes (hereafter, ‘song types’) having high variability: each song type included many song variants. Using playback experiments, we found that song versatility is an aggressive signal during male-male interactions in the black-faced bunting. We analysed four variables affecting song versatility: song-type diversity, syllable-type diversity, song-type switching rate and song-variant switching rate. First, we showed that all variables increased in the context of territorial competition. Then, we found that males reacted more strongly to versatile than to non-versatile stimuli. Finally, a male’s song-variant switching rate predicted its behavior during playback. We hypothesized that switching rate is the primary source of increasing versatility, and song and syllable diversity might be simply a consequence of increasing switching rate.</p>