The strength of sexual selection: a meta-analysis of bird studies

Sexual selection has been demonstrated to sometimes be strongly related to the expression of secondary sexual characters, as shown by a number of classical textbook examples, thereby providing evidence for the importance of sexual selection in the maintenance of secondary sexual characters, but equally many studies with no or only weak effects also exist. Because there is no general estimate of the magnitude of the relationship between intensity of sexual selection and expression of secondary sexual characters, we do not know to which extent extreme effects are typical, and whether there is an overall effect across studies. We made a meta-analysis of visual sexual signals in birds to test whether there is a general, significant relationship between the strength of sexual selection and the expression of secondary sexual characters and determined factors that accounted for some of the heterogeneity in effects among studies. The average effect size, measured as the Pearson product-moment correlation coefficient, was 0.30 for studies and 0.31 for species as units of analysis, which implies that 9-10% of the variance in male mating success is accounted for by intraspecific variation in the expression of secondary sexual characters. This finding is extremely robust given the high fail-safe number. We found some evidence consistent with publication bias, such as a decreasing effect with increasing sample size, whereas a decreasing variance in effect size with increasing sample size and the most common effect size being close to the average effect size did not suggest publication bias. Effect size was significantly negatively related to year of publication, suggesting that more representative studies have been published in recent years. Experimental studies demonstrated stronger effects than observational studies (weighted Pearson's r for experimental studies was 0.35), apparently because experimental studies increase the range of phenotypes and control for potentially confounding factors. Color signals did not differ in effect size from morphological structures. Monogamous and lekking species tended to show stronger effects than polgynous species (mean Pearson's r were 0.29, 0.48 and 0.23, respectively). Mean weighted effect size was larger for studies based on mating success than for studies based on mate preferences or reproductive success (mean Pearson's r were 0.35, 0.30, 0.28, respectively). A multiple regression analysis taking sample size into account demonstrated a significant effect of experiment and year of publication on effect size. Sexual selection on visual secondary sexual characters can thus be considered to have an intermediate effect on their maintenance.