Summary of species where the relationship between sperm morphology and paternity success has been assessed in the wild.

For each species, we report the between-male variation in sperm total length (expressed as the coefficient of variation, CVbm), the percent of offspring sired by an extra-pair male (EPY), the percent of broods containing at least one extra-pair offspring (EPB), and whether published works found significant relationships between sperm morphology and paternity. Where more than one published source is available for estimates of EPY and EPB, we preferentially use the estimate from the population and/or year where CVbm was assessed. Our sources are as follows: Tree swallow: [23, 65], Superb fairywren: [35, 66], Collared flycatcher: [37, 43], Bluethroat: this study, House wren: [36, 67, 68].