An adaptive explanation for the blue-green colouration of some avian eggshells has long interested researchers and naturalists alike. Unlike brown and maculated eggs that probably evolved through selection for crypsis, blue-green eggs would likely appear conspicuous to predators. It has been proposed that blue green egg colouration may be a sexually selected indicator of female quality. Because the pigment responsible for blue egg colouration is also an antioxidant, egg colour may act as a post-mating signal to the male of the laying female’s quality. Males may thus use egg colour to assess the female’s quality, and therefore, the quality of her offspring, investing more heavily in clutches of better quality females. The mountain bluebird (Sialia currucoides) is a socially monogamous species with biparental care, which lays blue eggs, making it an excellent species to test the sexual selection hypothesis for the evolution of blue egg colouration. Using digital photography, I examined the blue colouration of mountain bluebird eggs to investigate evidence for blue egg colouration acting as a sexually selected signal in mountain bluebirds. There were no consistent relationships between egg colouration and other variables, including parental provisioning rates, female condition, or offspring survival and growth rates. I conclude that there is insufficient evidence to support the notion that mountain bluebirds are using egg colouration as a sexually selected signal. However, variation in egg blue coloration was related to female provisioning rates, suggesting there may be a link between female quality and egg colouration.
