THE ROLE OF SONG RATE AND SONG BOUT'S COMPLEXITY IN THE TERRITORIAL BEHAVIOR OF RADDE'S WARBLER (PHYLLOSCOPUS SCHWARZI)

摘要

Birdsong is a highly complex signaling system as witnessed by its song/note type diversity and ordering rules. The question to ask is how the complexity of a birdsong is used in a species communication system. Males of many passerine birds sing actively during male-male interactions, including territorial intrusions. In this study, we asked whether the structure and organization of a song bout functions as a graded signal of aggression in Radde's warblers. Vocalization in Radde's warblers represents a strictly determined periodic process. All song types are produced according to fixed sequences which include 18-43 (median 38) different types. We simulated territorial intrusions by broadcasting Radde's warbler's songs within their territories. Experiments involved 19 different males. Each experiment had 3 consequential stages: "before playback" (5 min), "playback" (5 min), and "after playback" (5 min). The stages were described separately by means of 9 parameters: (1) median song length, (2) median pause length, (3) song rate (per min), (4) observed repertoire size, (5) song type switching rate. Song ordering rules was analyzed using transitional matrices. From the matrices, we calculated the sequence linearity score (S-LIN), sequence consistency score (S-CONS), stereotype score (S), and first-order relative entropy (RE1). The structure and organization of a song bout did not differ between "before playback" and "playback" stages, but all males clearly responded to a playback by approaching the loudspeaker and flying around. In contrast, comparing singing prior to with that produced after a playback, showed that (1) the pause length decreased, (2) the song rate increased, and (3) the observed repertoire size increased. Besides this, song type transition patterns during the "after playback" stage appeared to be more random as witnessed by decreasing S-LIN, S-CONS and S, and an increasing RE1. This study thus provides empirical evidence that the song rate and bout complexity (measured as the observed repertoire size and the parameters describing the ordering rules) could play a role in male-male competition in Radde's warbler.