Egg coloration is correlated with female condition in eastern bluebirds (Sialia sialis)

Egg coloration has been hypothesized to reflect female condition. Because of the proposed physiological costs associated with deposition of biliverdin pigments and because of their conspicuousness, eggs with blue-green coloration may reliably convey information about female or brood quality. We tested the hypothesis that expression of blue-green coloration of eastern bluebird (Sialia sialis) eggs positively correlates to female condition. First, we documented the incidence of egg color polymorphism within the population. We observed that 98% of females laid blue-green eggs while less than 2% laid white eggs and less than 1% laid pink eggs. In a subset of clutches, we used full spectrum reflectance spectrometry (300-700 nm) to compare eggshell coloration to measures of female condition. We found that the color of eggs within clutches was more similar than the color of eggs from different clutches, and that the blue-green eggs have spectral peaks that are consistent with the characteristic absorbance spectra of biliverdin pigmentation. Females in better body condition and older females laid more colorful eggs. Moreover, individual females laid more colorful eggs later in the laying sequence. Overall, these data indicate that egg coloration covaries with female condition, suggesting that egg coloration could function as a reliable signal of female quality or that egg coloration may allow females to recognize eggs laid by conspecific brood parasites.