Sexually selected egg coloration in spotless starlings.

It has been recently proposed that the blue-green coloration in eggs of many avian species may constitute a sexually selected female signal. Blue-green color intensity would reflect the physiological condition of females, and hence it might also affect the allocation of male parental care. In this study, we use three different experimental approaches to explore the importance of sexual selection on blue-green egg coloration of spotless starling (Sturnus unicolor) eggs. First, experimental deterioration of female body condition (by means of wing feather removal) negatively affected the intensity of blue-green egg coloration. Second, blue-green color intensity of artificial model eggs had a significant positive influence on paternal feeding effort. Finally, we found a negative relationship between the effect of experimental food supply on nestling immunocompetence and the intensity of blue-green coloration of eggs, suggesting that egg color predicts nutritional conditions that nestlings will experience during development. All these results taken together strongly support a role of sexual selection in the blue-green coloration of spotless starling eggs.