Anita. Behav., 1990, 40, 1158-1168 Song and information about aggressive responses of blackbirds, Turdus merula: evidence from interactive playback experiments with territory owners TORBEN DABELSTEEN* & SIMON BOEL PEDERSENt *Institute of Population Biology, Copenhagen University, Universitetsparken 15, D K-2100 Copenhagen O, Denmark t Electronics Institute, Technical University, DK-2800 Lyngby, Denmark 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 even 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. Male birds that start to sing on or near the territory of a conspecific probably constitute different degrees of threat to the owner. Several factors may contribute to the strength of the threat, e.g. the stage of the breeding cycle of the owner's mate, the singer's location and distance to the mate, which population the singer belongs to, its identity, its fighting ability and its 'intentions' (i.e. its ensuing behaviour and/or response). The response of the territory owner, therefore, should reflect the extent of shared information about such matters. Experiments mostly seem to show that territory owners do 'know' about their mates' reproductive state and take care of their own interests by responding most aggressively before and during the laying of eggs (e.g. Petrinovich et al. 1976; Dabelsteen 1984a). Owners also seem to consider information about the singer when this is provided by the song. For instance, they often respond more aggressively to song coming from the territory centre than to song from the territory boundary (e.g. Ickes & Ficken 1970; Melemis & Falls 1982), and to song of a stranger than to song of a neighbour (see Falls 1982). They may also respond differently to song from an alien and their own population (e.g. McGregor 1983; Hansen 1984). It is less clear whether owners might consider information about the singer's fighting capability and intentions (long- or short-term). The ability of song to communicate about fighting capability and long-term intentions is poorly known. On the other hand, a human observer may often extract infor- mation from a song about the singer's breeding condition, territoriality and general readiness to attack intruding rivals and/or court females. Such general or qualitative information about the poss- ible response of the singer should indicate to other males that they run a risk if they enter unfamiliar areas where a male is singing or do not expel singing males from their own territory. That territory owners of many species do respond aggressively to song in their territory and that males of certain 0003 3472/90/121158+ 11 $03.00/0 9 1990The Association for the Study of Animal Behaviour 1158