An experimental test of the effect of food and antioxidants on blue‐green eggshell coloration in mountain bluebirds Sialia currucoides

Evolutionary biologists have long debated the functional relevance of the ornamental traits that occur in many animal taxa, and yet, female ornaments have received relatively little attention compared to those produced by males. A greater understanding of these traits, particularly those that are unique to females, may shed light on the potential for sexual selection to shape female phenotypes. Recently, blue‐green eggshell colour, derived from the antioxidant pigment biliverdin, has been proposed as a candidate trait that advertises female quality to males in species of birds with biparental care. However, studies have been equivocal in their support for blue‐green eggshell colour being an informative signal, and correlations between eggshell colour and other female characteristics have been inconsistent. We conducted a supplementation experiment to test if improving the access of female birds to food resources and micronutrients, thereby improving their condition prior to egg laying, would intensify the blue‐green colouration of the eggs they laid. We provided mountain bluebirds Sialia currucoides with food and carotenoids during nest building and egg laying in two breeding seasons, and assessed both within‐ and among‐clutch variation in colour. Supplementation did not affect patterns of within‐clutch variation, but did result in differences in colour among clutches. Specifically, we found that food, but not carotenoids, resulted in higher colour saturation, and decreased the brightness of blue‐green eggshells. Although this trend was observed in both years, the effect was statistically significant only in one year. Our results suggest that food supplementation influences eggshell colour, but that conditions, such as weather and natural food availability, which can vary annually, may also determine how female birds allocate pigment to eggshells.