Song and information about aggressive responses of blackbirds, Turdus merula: evidence from interactive playback experiments with territory owners

Abstract

A combination of normal (one-way), interactive (two-way) and randomized ‘interactive’ (one-way) playback experiments were used to examine whether territorial blackbird males can use song to communicate varied information about their short-term intentions in agonistic situations. The study supports earlier analyses suggesting that low intensity song (LI) is least likely to be followed by attacks by the singer, whereas high intensity song (HI) is intermediate and strangled song (SS) is most likely to be followed by attacks. With normal playback, LI released the weakest, and HI and SS the strongest aggressive responses. Interactive playback, i.e. changing between LI, HI and SS in a natural way with the behaviour of the test bird, released evon stronger responses. Randomized ‘interactive’ playback, i.e. changing between LI, HI and SS independently of the behaviour of the test birds, released weaker responses than interactive playback. This indicates that the naturalness, rather than the variation of the sound stimulation, caused the strong responses to the latter. The results from interactive playback support the idea that SS indicates the most aggressive response of the singer. The escalations represented by the changes from HI to SS were apparent, however, only when the changes in song type were interactive. In other words, the interaction between the song type and the behaviour of the test bird was essential for the communicative value of SS. It is suggested that the adaptive significance of communicating such varied information about aggression lies in its relative safety and energy efficiency. Bluffing as a widespread strategy may have been prevented because the differences between the three types of song in production costs and how conspicuous they make the singer make bluffing more costly.