Anim. Behav., 1997, 53, 249–256 The signal function of overlapping singing in male robins TORBEN DABELSTEEN*, PETER K. MCGREGOR*†, JO HOLLAND*, JOE A. TOBIAS‡ & SIMON BOEL PEDERSEN* *Department of Population Biology and Centre for Sound Communication, Zoological Institute, University of Copenhagen †Behaviour & Ecology Research Group, Department of Life Science, University of Nottingham ‡Department of Zoology, University of Cambridge (Received 15 November 1995; initial acceptance 10 January 1996; final acceptance 2 April 1996; MS. number: 5079) Abstract. Songbirds can vary the timing of song production with respect to other singing individuals on a song-by-song timescale, for example birds may overlap songs or alternate singing and thereby avoid overlap. Playback was used to study the information contained in such timing of song exchanges in territorial male robins, Erithacus rubecula. The results are consistent with the idea that interacting with a singer either by overlapping or alternating is a way of indicating the intended receiver whereas non-interactive (loop) playback does not give this information. Furthermore, an overlapping pattern of singing generally elicited responses characteristic of highly aroused males. In robins this is shown by a rapid approach and change to an almost continuous, low amplitude pattern of singing referred to as twittering. Thus overlapping could be taken as indicating a high degree of arousal or a willingness to escalate. The response changed during the experimental period, with twittering responses becoming more common regardless of playback treatment. This result is consistent with experimental males having gathered information from interactions between playback and their neighbours in previous trials, that is, they collected information by eavesdropping.  1997 The Association for the Study of Animal Behaviour Songbirds are usually within hearing range of several other conspecifics in the local population and can therefore be considered as members of a communication network in which the individuals have a number of options for gaining information. They can listen to the widely broadcast full songs of other individuals, either by concentrating on specific individuals or by focusing their attention on their interactions (eavesdropping), or they can gain information in a more direct way through active participation in vocal interactions with each other (Dabelsteen & Pedersen 1991; Dabelsteen 1992; McGregor 1993; Dabelsteen & McGregor 1996; McGregor & Dabelsteen 1996). Concentrat- ing on a single individual implies that the receiv- ing bird extracts information from structural sound parameters such as frequency level, singing rate and sound type. Eavesdropping and active interaction with other individuals allow the added possibility of extracting information from inter- action phenomena such as interactive selection of the song types from the repertoire available and timing of the singing interactions. The infor- mation carrying capacity of structural parameters has been documented in several studies, for example, discrimination between species, popu- lations and individuals (e.g. Dabelsteen & Pedersen 1985, 1992, 1993; Nelson 1988; Weisman & Ratcliffe 1989; Weisman et al. 1990; see also Becker 1982 for a review). However, the signal value of the majority of interaction phenomena is more or less unknown because equipment for interactive playback experiments has only recently been developed and applied (e.g. Dabelsteen & Pedersen 1990, 1991; McGregor et al. 1992; Bradbury & Vehrencamp 1994; Otter et al. 1994; Dabelsteen et al. 1996). Correspondence: T. Dabelsteen, Department of Popu- lation Biology and Centre for Sound Communi- cation, Zoological Institute, Tagensvej 16, DK-2200 Copenhagen N, Denmark (email: tdabelstee@zi.ku.dk). P. K. McGregor is at the Department of Life Science, University of Nottingham, University Park, Nottingham NG7 2RD, U.K. J. A. Tobias is at the Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, U.K. 0003–3472/97/020249+08 $25.00/0/ar960369  1997 The Association for the Study of Animal Behaviour 249