Repeatability of sperm size and motility within and between seasons in the Barn Swallow (Hirundo rustica)

Sperm morphometry and motility are important male traits affecting fertility and post-copulatory competitive ability. However, studies examining consistency in sperm traits over time for individual males are rather scarce, particularly in free-living animals. Here, we report our analysis of various measures of sperm morphometry and sperm motility in the Barn Swallow (Hirundo rustica), both within and between breeding seasons. We found high within-season repeatabilities for all measures of sperm morphometry (all r > 0.70) and moderate repeatability of sperm motility (all r > 0.42). Between seasons, repeatabilities were somewhat lower, but total sperm length and flagellum length also showed very high repeatabilities between seasons (r > 0.85). Furthermore, sperm motility showed higher repeatabilities between seasons than within a season (all r > 0.63). Although measures of sperm motility are likely to be more plastic due to, for example, changes in body condition, they still showed moderate repeatabilities within and between seasons. Our study results also imply that a single ejaculate would be sufficient to obtain representative measures of an individual's sperm quality traits within the same breeding season.