Abstract
Postzygotic isolation may be important for maintaining species boundaries, particularly when premating barriers are incomplete. Little is known about the course of events leading from minor environmental mismatches affecting hybrid fitness to severe genetic incompatibilities causing sterility or inviability. We investigated whether reduced reproductive success of hybrid males was caused by suboptimal sperm traits or by more severe genetic incompatibilities in a hybrid zone of pied (Ficedula hypoleuca) and collared flycatchers (F. albicollis) on the island of Öland, Sweden. About 4 per cent hybridization is observed in this population and all female hybrids are sterile. We found no sperm in the ejaculates of most sampled hybrid males, and sperm with abnormal morphology in two hybrids. Furthermore, none of the hybrids sired any offspring because of high levels of hatching failure and extra-pair paternity in their nests. These results from a natural hybrid zone suggest that the spermatogenesis of hybrid males may become disrupted despite little genetic divergence between the parental species.