Sex-specific maternal effects in birds: mighty, but not almighty

In the last two decades, sex-specific maternal effects in birds have been a flourishing field of research. Yet, there are several well-established views that need new insight and potential revision. I will present methodological, empirical and meta-analytical examples of this kind. In the first study, I asked whether avian mothers differentiate investment into sons and daughters by laying sexually dimorphic eggs. To answer this question I have analysed egg sexual size dimorphism across 51 species and related it to the direction and to the degree of adult sexual size dimorphism in those species. In the second study, I investigated how increased levels of steroid hormones in the eggs of captive zebra finches influence early embryonic development of male and female offspring and whether maternal hormones influence sex-specific expression of the genes involved in prenatal development. Finally, I performed methodological meta-analysis which compared results of studies which manipulated egg hormone levels directly (by injecting the eggs) with those that used hormonal manipulation of mothers. I will present an essence of results steaming from 117 studies, including unexpected finding on the relationship between effect size and dose of manipulated hormone. In sum, topic of sex-specific maternal effects in birds is sexy, and as such, is well studied. This enables us to draw conclusions grounded on solid bases. When evidence is compelling, “no-significant” result is the new “sexy”.