All eggs are made equal: meta-analysis of egg sexual size dimorphism in birds

Sex-biased resource allocation in avian eggs has gained increasing interest. The adaptive explanations of such allocation are often related to life-history strategies of the studied species. In some species, egg sexual size dimorphism (SSD) was suggested to promote future size differences between adults of each sex. In other species, egg SSD was invoked as an adaptive means by which a mother balances sex-specific nestling mortality. According to the first scenario, mothers should produce bigger eggs for the bigger sex, thus across species, adult SSD should be a significant positive predictor of egg SSD. Under the second scenario, mothers should produce bigger eggs for the smaller sex. If different species use contrasting strategies, then a universal expectation is that there should be a significant relationship between the magnitude of adult SSD and the magnitude of egg SSD, irrespective of the direction of those differences. Our aim was to examine whether the direction of egg SSD is predicted by the direction of adult SSD or whether degree of egg SSD is related to degree of adult SSD. To answer that question, we performed meta-analysis of 63 studies, which included information on egg SSD of 65 effect sizes from 51 avian species. We found that across species, adult SSD does not predict egg SSD. More importantly, the observed variation in effect sizes in our data set was largely explained by sampling error (variance). Although adult SSD is undoubtedly a prominent feature of birds, there is little evidence for egg SSD across avian species.