1. Avian eggshell coloration has frequently been examined in a functional context (e.g. mimicry, camouflage), but in recent years, an interest has emerged in identifying the mechanisms that drive eggshell colour variation. 2. Eggshell coloration is predominately caused by pigment deposition; one such pigment is the antioxidant biliverdin, and deposition of biliverdin into eggshells may be costly to mothers due to depletion of their antioxidant reserves. Previous work has shown that dietary supplementation during laying with another type of antioxidant – carotenoid pigments – induces females to produce more biliverdin-rich eggshells. However, the impact of pre-laying nutrition – including the developmental period early in life – on eggshell coloration has not been investigated. 3. Here, we raised female mallards (Anas platyrhynchos) from hatching, supplemented their diets with carotenoids during early-, mid- or late-developmental periods, and at adulthood measured female circulating carotenoid levels, yolk carotenoid levels, and eggshell coloration. We found that carotenoid supplementation during the late stages of development (transitional period from juvenile to adult plumage) promoted the laying of eggs with more biliverdin-rich eggshells. Independent of developmental dietary treatment, females with higher circulating carotenoid levels at the time of egg laying produced more biliverdin-rich eggshells and more carotenoid-rich yolks. When controlling for female identity, we found that more biliverdin-rich eggshells were associated with more carotenoid-rich, but smaller, yolks. We also detected a laying order effect; later-laid eggs had larger, less carotenoid-rich yolks and less biliverdin-rich eggshells. 4. Taken together, these results demonstrate that eggshell coloration reveals carotenoid status of both mothers and yolks and that diet quality more than 1 month prior to laying can affect eggshell coloration in a waterfowl species. As mallards are considered to be capital breeders in terms of lipid stores, our findings provide a new developmental perspective on the carryover of lipid-soluble and antioxidant nutrient reserves for breeding.

Egg data by eggDryadData.xlsx