Speciation lies at the heart of evolution and the study of reproductive barriers allows a better understanding of the different steps leading to the complete isolation of two species. Pre-mating (behavior tactics, habitat or food divergence, phenotypic divergence and assortative mating) and post-mating, post-zygotic isolation barriers (selection against unfit hybrids) are well studied in numerous species, but little is known about what is happening between insemination and fertilization (post-mating, pre-zygotic isolation barriers). In this study, we chose the well-studied population of pied and collared flycatchers (Ficedula hypoleuca and F. albicollis) of the hybrid zone of Öland, Sweden, to investigate possible patterns of gamete divergence between these two closely related species. We compared sperm morphology between the two species and their hybrids, analyzing traits that are thought to play an important role in the fertilization success of the males. We did not detect any divergence in sperm morphology between the two species, but we report an extreme reduction of sperm production in hybrid males, as well as spermatogenesis dysfunctions and particularly high rates of extra-pair young in the nests of hybrid males.