Evidence of Hybridization between Black-Naped Tern (Sterna sumatrana) and Roseate Tern (Sterna dougallii) at St François Atoll, Seychelles

Incidence of hybridization, the interbreeding of species, is poorly-understood but believed to be comparatively prevalent in avian communities. In the Outer Islands of Seychelles, at St François Atoll, a Roseate Tern (Sterna dougallii) was first observed to have formed a pair with a Black-naped Tern in 2009. Subsequently, in 2015 conservation staff recorded an adult with intermediate characteristics, presumed to be a hybrid of the mixed-pair seen previously. The suspected hybrid proved fertile, backcrossing with the Black-naped Tern colony and producing offspring of its own. The resultant juvenile had red legs, exhibiting intermediate morphology. Presented here is an assembly of field observations and photographic evidence collected over 15 years which document mixed pairs; displaying defensive nest behavior, mate-feeding, incubating eggs, copulating and parenting chicks. Findings strongly indicate hybridization has taken place, which constitutes only the second example of such an event with this species combination worldwide. We also uncovered evidence for backcrossing, possibly compromising the genetic integrity of the Black-naped Tern colony which is of increased conservation value due to the relative scarcity of the mathewsi sub species.