Eggshell colour is more strongly affected by maternal identity than by dietary antioxidants in a captive poultry system

Abstract

Summary 1. Biologists have long puzzled over the apparent conspicuousness of blue‐green eggshell coloration in birds. One candidate explanation is the ‘sexual signalling hypothesis’ that the blue‐green colour of eggshells can reveal an intrinsic aspect of females' physiological quality, with only high‐quality females having sufficient antioxidant capacity to pigment their eggs with large amounts of biliverdin. Subsequent work has argued instead that eggshell colour might signal condition‐dependent traits based on diet. 2. Using Araucana chickens that lay blue‐green eggs, we explored (i) whether high levels of dietary antioxidants yield eggshells with greater blue‐green reflectance, (ii) whether females differ from one another in eggshell coloration despite standardized environments, diets and rearing conditions, and (iii) the relative strength with which diet vs. female identity affects eggshell coloration. 3. We reared birds to maturity and then placed them on either a high‐ or low‐antioxidant diet, differing fourfold in V itamin E acetate and V itamin A retinol. After 8 weeks, the treatments were reversed, such that females laid eggs on both diets in an order‐balanced design. We measured the reflectance spectra of 545 eggs from 25 females. 4. Diet had a very limited effect on eggshell spectral reflectance, but individual females differed strongly and consistently from one another, despite having been reared under uniform conditions. However, predictions from avian visual modelling suggest that most of the egg colour differences between females, and nearly all of the differences between diets, are unlikely to be visually discriminable. 5. Our data suggest that eggshell reflectance spectra may carry information on intrinsic properties of the female that laid the eggs, but the utility of this coloration as a signal to conspecifics in this species may be limited by the sensitivity of a receiver to detect it.