The saltmarsh sparrow Ammospiza caudacuta and Nelson's sparrow A. nelsoni differ in ecological niche, mating behavior, and plumage, but they hybridize where their breeding distributions overlap. In this advanced hybrid zone, past interbreeding and current backcrossing result in substantial genomic introgression in both directions, although few hybrids are currently produced in most locations. However, because both species are nonterritorial and have only brief male-female interactions, it is difficult to determine to what extent assortative mating explains the low frequency of hybrid offspring. Since females often copulate with multiple males, a role of sperm as a postcopulatory prezygotic barrier appears plausible. Here, we show that sperm length differs between the two species in the hybrid zone, with low among-male variation consistent with strong postcopulatory sexual selection on sperm cells. We hypothesize that divergence in sperm length may constitute a reproductive barrier between species, as sperm length co-evolves with the size of specialized female sperm storage tubules. Sperm does not appear to act as a postzygotic barrier, as sperm from hybrids was unexceptional.
