Condition-dependent strategies of eggshell pigmentation: An experimental study of Japanese quail (Coturnix coturnix japonica)

Abstract

A relationship has been suggested between eggshell colour and female body condition based on the opposing antioxidant properties of the two main eggshell pigments: the antioxidant biliverdin (blue–green) and the pro-oxidant protoporphyrin (brown). We hypothesized that experimentally food-restricted females with low antioxidant capacity would deposit more protoporphyrin and less biliverdin in their eggshells, resulting in eggshells of reduced brightness but increased colour intensity. Two eggs were collected at the beginning and two at the end of a 2 week period from each of 24 female Japanese quails that were either food restricted or receiving ad libitum food (i.e. controls) during that time. Reflectance spectra were recorded and analysed using spectral shape descriptors, chromatic and achromatic contrasts were computed accounting for avian visual sensitivities, and eggshell pigments were quantified. We examined both spot and background pigmentation and found no significant effect of food restriction on eggshell reflectance. However, food-restricted females in lower body condition increased the deposition of protoporphyrin and decreased the amount of biliverdin invested in their eggshells. We hypothesize that in species laying brown-spotted eggshells, females modulate eggshell pigment investment in response to their body condition. According to this hypothesis, we predict that females maintain eggshell colour to limit visible changes that could be detected by predators and thereby conceal their eggs, although this work has yet to be conducted. We suggest that further experimental work on egg camouflage under different environmental conditions will elaborate on the process of pigment deposition and the physiological costs to females of laying heavily pigmented eggshells.