A study by Ph.D. student Hanna Nyborg Støstad has investigated the peculiar spiral shape of songbird sperm. Støstad compared sperm cells of 36 bird species including house sparrows and tree swallows, and found that species whose sperm had a particularly distinct spiral or corkscrew shape also had sperm with high average swimming speed. However, those species also tended to have high rates of abnormal or damaged sperm.