Abstracts (first author)
All eggs are made equal
Maternal effects mediated by egg size may have profound effects on offspring fitness. Sex-biased resource allocation in birds gains increasing interest, but it is not known to what extent the egg sexual size dimorphism (SSD) is a wide-spread phenomenon in this group. To answer that question we performed meta-analysis of 33 published and 2 unpublished studies, which included information on egg SSD of 31 avian species. Many of those studies suggested adaptive explanation for the reported egg SSD, which helped us to formulate predictions for our analyses. In some species, egg SSD was suggested to promote future size differences between the adults. If that is the case, then across species, adult SSD should be a significant predictor of egg SSD. However, in other species, egg SSD was invoked as an adaptive means by which a female balances nestling mortality differences between sexes, therefore producing bigger eggs for the smaller sex. Based on these two hypotheses, we derived a general prediction 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 analyses found no support for either of those hypotheses. Across species, adult SSD does not predict egg SSD. More importantly, our meta-analysis revealed no heterogeneity, with the meta-analytic mean very close to 0. That is, the observed variation in effect sizes in our dataset was almost exclusively explained by sampling error and there was no difference in avian egg sizes between the sexes whatsoever. Although adult SSD is undoubtedly a prominent feature of avian species, we conclude that, in general, there is no evidence for egg SSD across bird species.