In the sympatry zone many Pied Flycatcher males perform songs that resemble those of a sibling species (Collared Flycatcher): these are so-called mixed songs. The higher abundance of the Collared Flycatcher was once considered a possible cause of mixed singing. To examine this hypothesis we studied the influence of the Collared Flycatcher on the interpopulational song variability in the Pied Flycatcher in four areas with different abundances of Collared Flycatchers. We focused on: 1) the abundance of mixed singers and their distribution; 2) geographical variations of typical Pied Flycatcher songs. We show for the first time that mixed singing in the Pied Flycatcher is common also when its abundance is as high as and higher than that of the Collared Flycatcher. In the old area of sympatry about 40% of all Pied Flycatcher males had Collared-Flycatcher-like syllables in their repertoire. At the same time, the habitat distribution of mixed singers matches that of the Collared Flycatcher. In the context of our data we propose a hypothesis of mixed song-formation. We have also demonstrated interpopulational variability in pure Pied Flycatcher songs, with species-specific differences being more pronounced in sympatric populations than in allopatric ones. This pattern is in agreement with the theory of character displacement and acoustic divergence in a sympatry zone, but the differences discovered in pure vocalizations are evidently not the result of interactions with the sibling species.