Song in birds is thought to function mainly as an intraspecific signal. But it can also be used in interspecific communication, among others in cases of interspecific territoriality. However, the function and information content of interspecific singing is not well studied. We recorded song responses of territorial black-capped warbler S. atricapilla males to playbacks of conspecific song and to the garden warbler's S. borin song. A preliminary analysis suggested that some blackcap warbler males, while responding to the garden warbler's song, can alter phrasing of their song to resemble the song of the congener. We present quantitative analyses of sonograms and discuss potential significance of such behaviour.