The first record of a female hybrid between the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (Luscinia luscinia) in nature

Understanding the mechanisms causing reproductive isolation between incipient species can give important insights into the process of species origin. Here, we describe a record of a hybrid female between two closely related bird species, the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (L. luscinia). These species are separated by incomplete prezygotic isolation, and the occurrence of fertile hybrid males in nature has been documented before. Our record represents the first genetically confirmed evidence of the occurrence of hybrid females between these two species in nature. Although the hybrid female was captured in the peak of the breeding season in suitable habitat, it did not show any sign of reproductive activity, suggesting that it was sterile. This observation together with previous experimental work in captivity suggests that hybrid female sterility plays an important role in reproductive isolation in nightingales. Unexpectedly, the hybrid female had highly developed fat reserves and was in the process of moulting, which in nightingales normally occurs approximately 1 month later. This unusual moult pattern could contribute to lower fitness of hybrid females and thus to speciation in nightingales.