Behavioural Processes 46 (1999) 151 – 158 Maintenance of male reaction to the congeneric song in the Hippolais warbler hybrid zone J. Secondi a, *, B. Faivre a , M. Kreutzer b a Laboratoire Ecologie -Eolution, Uniersite ´ de Bourgogne, 6, Bld. Gabriel, 21000 Dijon, France b Laboratoire de Psychophysiologie et d’e ´thologie, Uniersite ´ Paris X. 200, Aenue de la Re ´publique, 92001 Nanterre Cedex, France Received 30 September 1998; received in revised form 25 February 1999; accepted 1 March 1999 Abstract Both hybridization and competition may induce character shift on communication systems in hybrid zones, leading either to convergence or divergence. Melodious warblers Hippolais polyglotta and Icterine warblers H. icterina are interspecifically territorial and hybridize along a narrow contact zone. This spatial distribution is favoured by a reaction to the congeneric song. A character displacement, e.g. a decrease of the reaction, was expected within the hybrid zone because of reproduction costs lower than in allopatry. We investigated the pattern of variation of the interspecific reaction by comparing allopatric and sympatric populations. However, our data did not support the hypothesis character displacement and we propose several alternatives to account for the maintenance of interspecific reaction in sympatry. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Competition; Hybridization; Song learning; Territoriality; Moving front 1. Introduction Reaction to heterospecific signals can be fa- voured under various ecological conditions, rang- ing from cooperation (Moynihan 1968; Hurd, 1996) to competition (Cody 1973; Catchpole 1978; Catchpole and Leisler, 1986; Martin et al., 1996). In the latter case, the reaction to a heterospecific signal will often depend on local ecological condi- tions. For instance, Great Tits Parus major and Chaffinches Fringilla coelebs have been reported to actively respond to songs of the other species in an insular environment, whereas no such response was reported for their mainland counterparts ex- ploiting a more heterogeneous environment (Reed, 1982). In the case of closely-related species that are active at the same season and in the same environment, reaction to heterospecific signals may correspond to competition (Orians and Will- son, 1964; Rohwer, 1972; Catchpole, 1978; Rice, * Corresponding author. 0376-6357/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII:S0376-6357(99)00029-7