Singing direction as a tool to investigate the function of birdsong: an experiment on sedge warblers

Directional communication, in other words the use of body orientation to increase transmission of directional signals, has been widely neglected in studies of animal communication. Here we propose that the experimental analysis of singing direction in birds can provide insights into the function of song. We studied three populations of sedge warblers, Acrocephalus schoenobaenus, in Austria, Britain and Germany. In this species, song is thought to be primarily a mate attraction signal, as males cease to sing once they are paired. Using behavioural observation during playback experiments, we found that males approached a loudspeaker simulating a rival male and also oriented themselves towards the loudspeaker while singing. This finding shows that sedge warbler songs are also directed at rival males and that the dual functions of mate attraction and territory defence were only hidden from the human observer because singing ceases after pairing. We argue that the study of directed communication is an additional tool in addressing functional questions, and that animal orientation should be taken into account when conducting investigations of signalling behaviour.