Slow song syllable rates provoke stronger male territorial responses in Eurasian Reed Warblers (Acrocephalus scirpaceus)

In birds, male song is mainly driven by sexual selection, whereby several song characteristics have been demonstrated to be important. In this context, the number of syllables produced per time unit (syllable rate), which thus reflects the "speed" of a song, has been suggested as a reliable indicator of male quality. It has been also shown that syllable rate reflects the aggressive state of an individual. We examined the importance of syllable rate for male-male interactions in unmated, territorial Eurasian Reed Warbler (Acrocephalus scirpaceus) males, for which song plays an important role in territory defense and mate choice, but there is a lack of information about which characteristics might be specifically important for territorial defense. We investigated the behavioral response of the receiver to varying syllable rates. Playback experiments were performed with two playback stimuli representing high and low syllable rates, which were presented simultaneously within the territory of an unmated male. The response of the territory owner was determined using several behavioral parameters and revealed to differ significantly between the two playback treatments with varying song speeds. More males approached first and came closer to the sound of the slow song as well as staying nearby longer. This reaction might indicate that the receiver is intimidated by the more aggressive nature of the fast song type.