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 clas sical textbook examples, thereby providing evidence for the importance of s exual selection in the maintenance of secondary sexual characters, but equa lly 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 no t 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 si gnals in birds to test whether there is a general, significant relationship between the strength of sexual selection and the expression of secondary s exual characters and determined factors that accounted for some of the hete rogeneity in effects among studies. The average effect size, measured as th e 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 vari ance in male mating success is accounted for by intraspecific variation in the expression of secondary sexual characters. This finding is extremely ro bust given the high fail-safe number. We found some evidence consistent wit h 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 rela ted to year of publication, suggesting that more representative studies hav e been published in recent years. Experimental studies demonstrated stronge r effects than observational studies (weighted Pearson's r for experimental studies was 0.35), apparently because experimental studies increase the ra nge of phenotypes and control for potentially confounding factors. Color si gnals did not differ in effect size from morphological structures. Monogamo us and lekking species tended to show stronger effects than polgynous speci es (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 studie s based on mate preferences or reproductive success (mean Pearson's r were 0.35, 0.30, 0.28, respectively). A multiple regression analysis taking samp le size into account demonstrated a significant effect of experiment and ye ar of publication on effect size. Sexual selection on visual secondary sexu al characters can thus be considered to have an intermediate effect on thei r maintenance.