Capsule Sister taxa hybridize more frequently than non-sister taxa. Aims To test whether the frequency of hybridization is higher in avian sister species than in non-sister species, based on molecular phylogenetic relationships and reports of hybridizing bird species globally. Methods A literature based survey of hybridizing bird species was conducted on genera that: (1) have a completely known phylogenetic molecular tree for at least 90% of the total number of species, (2) have at least four species, (3) have an incidence of hybridization more than 25% recorded from the wild, and (4) have at least two pairs of hybridizing species. The frequency of hybridization for avian sister species was compared to that of non-sister species. Results Twenty-nine genera were identified that met our four selection criteria. In 25 genera, sister species hybridized more than non-sister species (mean frequencies of 0.52±0.35 versus 0.16±0.13). Conclusion The frequency of hybridization within sister species was found to be higher than within nonsister species.

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