In this study I examined intra-specific variation in primary sex traits in male bluethroats (Luscinia svecica svecica), a passerine species with a high intensity of sperm competition, The single most important predictor of the size of testes and seminal glomera was male age (yearlings versus older). This finding suggests that older males have higher sperm production rates, which may allow for higher copulation rates and/or larger ejaculates than in younger males. Previous findings of older males having a higher extra-pair fertilization success, and similar paternity loss in own nest as younger males despite less intense mate guarding, contribute to a general pattern of age-dependent sperm competition investments in male bluethroats. None of the measured conditional variables (body mass, haemoglobin, haematocrit), nor the body size variables (wing length, tarsus length, scull length) correlated significantly with testis size. The bluethroat sperm were relatively long (216.4 µm, n=46, SD=3.1µm) as expected for a species with intense sperm competition. Between-male variation in average sperm length was considerably larger than the within-male variation in individual sperm length. However, the between-male coefficient of variation in mean sperm length was considerably lower than that reported recently for another passerine with low intensities of sperm competition. More species should be examined to test for a possible relationship between inter-male variation in sperm length and the intensity of sperm competition
