This bird family contains small, insectivorous songbirds that occur mainly in Eurasia, African and the Australasian region. Hybrids are known in one genus, Locustella.
In Eastern Poland, a bird was recorded that sung River Warbler (L. fluviatilis) and Grasshopper Warbler (L. naevia) songs. The most likely explanation seemed to be vocal mimicry, but hybridization cannot be ruled out (Polakowski et al., 2013). Interestingly, a hybrid between these two species is as yet unknown.
Polakowski, M., Broniszewska, M., Jankowiak, L. & Cofta, T. (2013). Hybridisation or vocal mimicry? A case of a mixed singing River Warbler Locustella fluviatilis in eastern Poland. Turkish Journal of Zoology 37, 246-248.