Studies of acoustic communication in animals focus mostly on conspicuous signals, i.e. loud signals, whereas the use of more unobtrusive signals seems to somehow be overlooked. However, communication network theory predicts the use of quiet signals as a common strategy to counter eavesdropping. Quiet song may both secure privacy of the communication and make the singers anonymous. This may be an advantage if e.g. a bird looses a fight because observers could gain information about its low quality. Male common blackbirds (Turdus merula) have different singing styles: the loud full song and a quiet twitter song, the latter being sung in certain contexts, e.g. aggressive interactions. The twitter song resembles the terminating twitter of full songs in being quiet and broadband. The high frequencies increase privacy as such signals attenuate strongly with increasing distance. The aggressive twitter song seems furthermore to be characterised by a larger repertoire compared to the full song which may make individual identification more difficult. Both the low volume and the large repertoire make the twitter song a good candidate for an unobtrusive signal which also increases the chances of staying anonymous. In this study, we conducted playback experiments in which we play full song of conspecifics to territorial birds, provoking aggressive twitter song by simulating an intruder. We analyse the obtained aggressive twitter and compare it with that of the full song to determine the degree of overlap between both singing styles. We discuss whether the results support the theory of privacy and anonymity in communication networks.
Abstract: