THE SIGNIFICANCE OF A 2 PART SONG IN THE MOUSTACHED WARBLER (ACROCEPHALUS-MELANOPOGON)

There are several ways of timing male and female oriented messages thr ough the song of a male bird. We examined a case where both functions are accomplished in one song type but seem to be separated into differ ent song parts. Therefore, we firstly tried to confirm the functional separation of the male and female directed parts through recording the natural occurrence of the two song parts and secondly we tried to eva luate possible benefits of such a two part song by means of playback e xperiments. The song of male moustached warblers Acrocephalus melanopo gon consists of a reed warbler-like warbling part often initiated by a repetiton of one simple whistle note (whistle part). According to the general intra-intersexual hypothesis, specifically developed for Acro cephalus warblers, the simple stereotyped beginning (whistle part) sho uld be directed to male competitors and the complex ending (warbling p art) to females. We obtained verification of this hypothesis through b ehavioural observations under natural conditions. For instance, the pr oportion of warbling song decreases after mating and the relationship between length of warbling song and mating interval - the days between arrival until mating of males - suggests that the warbling part is im portant for female attraction. The whistle part decreases less steeply after mating and actually increases significantly during the post-fer tile period. Furthermore, the occurrence of whistling is related to pr edator disturbance and intraspecific territorial intrusions. Concernin g possible benefits, by offering different playback types to territory holders, fewer males attacked the speaker if whistle intensity increa sed. So, there seems to be a clear benefit of introducing the song wit h a male oriented message. The same advantage seems to hold in intersp ecific context with the reed warbler which was also tested by playback experiments.