Eggshell biliverdin concentration does not sufficiently predict eggshell coloration

Avian eggshell coloration may have arisen due to selection on the biological, chemical, or physical properties of the pigments embedded within the eggshell, or due to selection on the coloration that emerges due to pigment deposition. Within both hypothetical frameworks, pigment‐based eggshell coloration would be related to metrics of egg quality; however, no one has evaluated the relative strength of coloration and pigment concentration in predicting egg quality. Here, we examined 66 European starling Sturnus vulgaris eggs and quantified eggshell biliverdin concentration (an antioxidant that produces the eggshell's blue coloration) and used 28 different coloration metrics derived from both photographic and spectrophotometric data. We also measured egg size, eggshell thickness, concentration of carotenoids in the yolk, and concentration of lysozyme in the albumen to capture variation in egg quality. We found that throughout the laying sequence, biliverdin concentration increased while eggshell thickness, yolk carotenoid concentration, and lysozyme concentration in the albumen all decreased, but this variation was not captured by any eggshell coloration metric. Both eggshell coloration and biliverdin concentration were negatively associated with yolk carotenoid concentration, but only eggshell biliverdin concentration was negatively associated with yolk mass. Lastly, biliverdin concentration explained, at most, only 46% of the variation for all eggshell coloration metrics. Our results suggest biliverdin concentration is a better predictor of egg quality than egg coloration in European starlings, supporting the hypothesis that eggshell pigment concentration per se may be the target of selection.