Hippolais polyglotta and H. icterina are two bird species whose parapatric distributions slightly overlap. The sympatric zone has moved during last decades because of the breeding range expansion of the former species and of the regression of the latter. Both species hybridise and evidence of introgression for morphological characters has been observed in an old zone of sympatry. Only the receding species H. icterina exhibited a morphological shift, suggesting that Melodious warblers asymetrically introgressed into Icterine warblers. Song analyses were carried out on males from sympatric and allopatric areas. This study aimed at assessing the congruence of morphological and vocal patterns of variation within the hybrid zone. 80th species are interspecifically territorial so that competition for space could be a major component of song evolution. In addition, song is learned in oscines. Thus, the patterns of variation of morphology and signal could differ to some extent. A parallel evolution was found for Icterine warblers, suggesting an influence of hybridisation. However, a vocal shift occurred in sympatric H. polyglotta where no morphological evidence of hybridisation was observed. Hybridisation alone cannot explain song variations in Hippolais warblers. Epigenetic and other evolutionary mechanisms are likely involved in producing the observed pattern. Finally, the convergence in signals in sympatry could increase the occurrence of hybridisation and partially explain the dynamics of the hybrid zone.
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