Seasonal variation in sex ratio and sexual egg dimorphism favouring daughters in first clutches of the spotless starling : Seasonal sex ratio trends and egg dimorphism

Good-genes hypotheses of sexual selection predict that offspring fathered by preferred males should have increased viability resulting from superior genetic quality. Several studies of birds have reported findings consistent with this prediction, but maternal effects are an important confounding variable. Those studies that have attempted to control for maternal effects have only considered differential maternal investment after egg laying. However, female birds differentially deposit testosterone in the eggs, and this influences the development of the chick. This study shows that female birds deposit higher amounts of testosterone and 5alpha-dihydrotestosterone in their eggs when mated to more attractive males.