Breeding Success of Interspecies Hybrids: Reduced Fertility in a Hybrid Magpie Population (Pica pica × Pica serica, Aves)

Data on the breeding success of hybrids, obtained by means of direct observations in nature, are key to understanding reproductive barriers between species and predicting the fate of hybrid populations. Limited hybridization occurs in an emerging secondary contact zone between the Eurasian magpie Pica pica and Oriental magpie Pica serica. This was identified during our recent study of single nucleotide polymorphisms of nuclear DNA. Here we present comparative analysis of the composition of 119 breeding pairs and content of 89 nests, found in the zone of recent contact between P. pica leucoptera and P. serica jankowskii in Northeastern Mongolia and Eastern Transbaikalia, as well as in relatively pure populations in adjacent regions. In the Mongolian hybrid population we recorded higher proportions of completely non-viable clutches, as well as partially viable clutches, as compared to those of pure P. p. leucoptera or P. s. jankowskii populations. Hybridization seems less intensive in Eastern Transbaikalia, where the proportion of completely non-viable clutches, as well as the proportion of partly dead clutches, is much lower than in Mongolia. Several possible mechanisms of genetic incompatibilities and the breaking of the postzygotic isolation are discussed. The future fate of this hybrid contact zone is considered, taking into account selection against hybridization and possible reinforcement of prezygotic isolation, including differences in species-specific vocalizations. A mosaic hybrid zone with features of a “tension hybrid zone” may develop here. This unique observation of an emerging zone of contact and hybridization of two young magpie species deserves ongoing monitoring and genomic studies.