During May and June 2003, a mixed breeding pair consisting of a male western Bonelli's warbler (Phylloscopus bonelli) and a female wood warbler (P. sibilatrix) successfully raised six young in a broad-leafed forest near Jü nkerath, western Germany, an area far outside the regular breeding range of P. bonelli. The identity of the adult birds was confirmed by song, calls, colouration, and wing measurements. Based on blood samples taken from both parents and three juveniles, the potential interbreeding was analysed by molecular methods. Sequences of the mitochondrial cytochrome b gene differed between male and female by 8.7% and confirmed the identification of both parents. Sequences of the mitochondrial cytochrome b gene revealed that the male was a western Bonelli's and the female a wood warbler. The hybridisation and the parentage of male and female were corroborated by multilocus DNA fingerprinting. This is the first documented and genetically proven hybridisation event between these two warbler species.

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