The A zores bullfinch ( P yrrhula murina ) has the same unusual and size‐variable sperm morphology as the E urasian bullfinch ( P yrrhula pyrrhula )

The A zores bullfinch is endemic to the island of S ão M iguel in the A zores archipelago and the sister species to the E urasian 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 E urasian 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 A zores 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 A zores bullfinch has retained more diversity at functional and fitness‐related loci than at more neutral parts of the genome. Finally, we also present data on relative testis size and sperm swimming speed for the E urasian bullfinch, and discuss the hypothesis that the small and putatively neotenous sperm in bullfinches has evolved in response to lack of sperm competition. © 2013 The Linnean Society of London