The Azores Bullfinch (Pyrrhula murina) has the same unusual and size-variable sperm morphology as the Eurasian Bullfinch (Pyrrhula pyrrhula)

The Azores bullfinch is endemic to the island of São Miguel in the Azores archipelago and the sister species to the Eurasian bullfinch. Here we show that the spermatozoa of the two species have similar ultrastructure and gross morphology. Thus, the unusual and supposedly neotenous sperm morphology previously described for the Eurasian bullfinch appears to be an ancestral trait that evolved before the two taxa diverged. In addition, the coefficients of variation in total sperm length, both within and among males, were high in both species and exceed any previously published values for free-living passerines. Such high sperm-size variation is typically found in species with relaxed sperm competition. However, the high variance in mean sperm length among Azores bullfinches is surprising, because the trait has high heritability and this small, insular population shows clear signs of reduced genetic diversity at neutral loci. A possible explanation for this apparent contradiction is that the...