The sperm of the Eurasian Bullfinch (Pyrrhula pyrrhula) differs markedly in gross morphology from that of all other passerines examined to date. In other passerines, the sperm head is pointed and helical, and the midpiece comprises a mitochondrial helix extending along the flagellum; whereas in the Eurasian Bullfinch, the sperm acrosome is rounded, not helical, and the midpiece is extremely short. In a pairwise study, using principal component analysis (PCA), we combined quantitative and qualitative sperm morphology traits and conducted a phylogenetic correlation to compare the sperm morphology of Eurasian Bullfinch and Beavan's Bullfinch (P. erythaca) with nine other pairs of congeneric passerines. The analysis revealed that Eurasian Bullfinch was a dramatic outlier in sperm morphology and that Eurasian and Beavan's bullfinches are more different than any other pair of species. Excluding Eurasian Bullfinch from the analysis showed that most variation in sperm morphology in the other species was attributable to phylogeny. The Eurasian Bullfinch also has extremely small testes for its body size, which indicates that sperm competition is infrequent in this species; we discuss the possibility that relaxed selection, via lack of sperm competition, may have contributed to the species' unusual sperm morphology.

Morfología Espermática Inusual en Pyrrhula pyrrhula