Moving hybrid zones are receiving increasing attention. However, so far little is known about the proximate mechanisms underlying these movements. Signalling behaviour, by individuals engaged in interspecific sexual and aggressive interactions, may play a crucial role. In this study, we investigated song variation within a moving hybrid zone between two warblers, Hippolais polyglotta and H. icterina. In these species, song is involved in interspecific territoriality and, probably, in mixed pairings. We showed that allopatric populations of the two species are clearly acoustically differentiated. However, interspecific differences faded out in sympatry as a result of an overall pattern of convergence. Unexpectedly, the two species converged for different song parameters, namely temporal parameters for H. icterina and syntax for H. polyglotta. Hybridization and interspecific competition could explain convergence in H. icterina. Instead, in H. polyglotta we suggest that local adaptation to habitat and interspecific learning might contribute to convergence. We particularly stress that cross-species learning, by maintaining high levels of interspecific interactions, may influence the movement of the zone. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society, 2003, 80, 507–517.