Extra-pair paternity may drive selection on spermatozoa and ejaculate characteristics through sperm competition and cryptic female choice. Here, we examine sperm morphology in the black-throated blue warbler (Setophaga caerulescens), an ecological model species where extra-pair paternity is frequent and is linked with male age. We test whether sperm morphology relates to several aspects of male phenotype known or suspected to affect extra-pair paternity success. Sperm morphology did not correlate with the size of the white wing spot, a social status signal, nor with the volume of the cloacal protuberance. However, older males tended to have longer sperm cells. Although the sample size was limited, this pattern is intriguing, as longer cells may be advantageous in post-copulatory sexual selection and older males have larger testes and higher extra-pair paternity success in this species. Changes in sperm morphology with age are not observed in other birds, though they have been observed in insects and fishes. More research on sperm morphology is needed to clarify its role in extra-pair fertilizations in this well-studied species.
