The Pied and Collared Flycatchers Ficedula hypoleuca and F. albicollis hybridize on the Baltic islands of Oland and Gotland. Field studies of hatching success in clutches from 36 breeding hybrid males indicate that male hybrids reproduce successfully. In comparison, 25 hybrid females had normal clutch sizes but failed to hatch nestlings, suggesting female hybrid sterility. The paternity of seven hybrid males was investigated by DNA fingerprinting using two hypervariable minisatellite probes. Of these hybrid males, six were assessed as fertile. Three of the hybrid male families (43%) contained nestlings with extra-pair paternity and the frequency of extra-pair fertilization among 37 nestlings was 22%. One nestling originated from intraspecific female nest parasitism. The sterility pattern in hybrids between these two flycatchers with sterile female and fertile male hybrids is in agreement with Haldane's rule.