Biliverdin- and protoporphyrin-based eggshell pigmentation in relation to antioxidant supplementation, female characteristics and egg traits in the canary (Serinus canaria)

Abstract

Abstract: Avian eggs exhibit a large variability in coloration and patterns, which are produced by blue-green biliverdin and red-brown protoporphyrin pigments. Several hypotheses have been proposed to explain the function of eggshell coloration. In this experimental study, we tested two hypotheses (signalling-function hypothesis and structural-function hypothesis) on both eggshell pigment types in an open-nesting songbird, the canary (Serinus canaria). Also, we aimed to examine whether deposition of pigments into the eggshell has any cost in terms of the plasma oxidative status of the female. We found that eggshell average blue-green chroma was increased by antioxidant supplementation, although we note that there had already been a pre-existing bias in plasma antioxidant capacity between the supplemented and control groups. Eggshell average blue-green chroma was positively related to female body condition during egg laying. However, blue-green eggshell colour was not related to female oxidative status during or after the laying period, and blue-green chroma increased with laying order. Accordingly, we found some support for that eggshell blue-green colour could reflect maternal antioxidant availability and body condition, but did not find evidence that it has a cost for the female’s oxidative status. By contrast, eggshell spot brightness was positively related to body condition, suggesting that darker spotting reflected poorer nutritional condition in the canary. Eggshell blue-green pigmentation was not significantly connected to the egg volume or yolk antioxidant level, but we found that eggs with lower yolk antioxidant concentration had higher average eggshell brown chroma. In sum, our results suggest that eggshell colour reflected female antioxidant and nutrient availability. Finally, we found that eggs with thinner eggshells had a more aggregated spot distribution, supporting the view that aggregated spots may help to strengthen eggshells. Significance statement: Avian eggs have a large variability in colours and patterns, which are due to two pigments: the blue-green biliverdin and the red-brown protoporphyrin. In this study, we tested whether eggshell pigmentation, measured by a spectrophotometer, could reflect female and egg quality in the canary, an open nesting songbird. We found that females supplemented with antioxidants before and during egg-laying laid more intense blue-green eggs. Females in better body condition laid eggs that had a more intense blue-green coloration, but lower intensity of brown coloration. Egg yolk antioxidant level was lower in eggs with more intense brown eggshell coloration. These results suggest that eggshell coloration could reflect the antioxidant and nutrient availability of females. Moreover, we found that eggs with thinner eggshells had a more aggregated spot distribution on the eggshell, supporting the view that aggregated brown pigment spots may help to strengthen eggshells.