Sex-Specific Yolk Allocation Revisited: A Test in Starlings and a Review of Evidence in Birds

Maternal effects play a key role in shaping offspring phenotype variation in birds, particularly through the allocation of different substances into the egg yolk. Among these, androgens and antioxidants are essential for early development. These compounds may exert sex-specific effects, as male and female offspring often differ in their development even from very early phases of their ontogeny. It has been therefore hypothesized that females might strategically allocate different amounts of these yolk compounds depending on the sex of the embryo. To test this hypothesis, we conducted an observational study in a wild population of spotless starlings (Sturnus unicolor), measured yolk concentrations of androgens (testosterone and androstenedione) and antioxidants (vitamin E, vitamin A, and carotenoids) in freshly laid eggs, and molecularly sexed the embryos from those eggs. We found no evidence that embryo sex influenced the concentration of any yolk compound. These results suggest that sex-specific allocation is constrained, possibly due to mechanistic limitations, such as the tight temporal overlap between oocyte meiosis (when embryonic sex is determined) and yolk formation. We also conducted a systematic literature review of studies that tested the association between embryo sex and yolk androgens and/or antioxidants in birds. Most studies (73%) did not report any significant effect. Among those that did, effects were often context-dependent or potentially biased by the effect of embryonic metabolism due to late sampling after laying. Overall, we found no robust evidence supporting differential maternal allocation of these yolk compounds based on embryo sex.