Abstract
Although egg color is generally consistent within individual birds and robust to environmental variation, recent evidence suggests a degree of susceptibility to environmental perturbation or modulation of egg color. Most of this variation manifests via the physiology of the laying female, but some direct impacts of the environment on laid eggs have also been discovered. Here we test whether light changes bird egg color and we quantify its effect, by subjecting variable blue-green eggs of Rüppell's weaver (Ploceus galbula) to a broad-spectrum light source under laboratory conditions, and measuring egg reflectance every few hours. Eggshells gradually decreased in reflectance across the entire measured wavelength spectrum of 250–800 nm. Reflectance peaks were disproportionately affected, such that the height decreased of both the blue-green peak and the smaller UV peak typical of blue-green eggs. The reflectance of lighter eggs was affected slightly more than that of darker eggs. These changes are similar to previous results for changes over long periods of time in darkness, suggesting that light might hasten the same process of pigment degradation that proceeds even without light. Comparison between the experimental light source and both sunlight and typical artificial lighting situations raises the possibility that significant color change might occur during incubation in some birds, but indicates that eggshell illumination in museums for short periods of study is unlikely to affect their color to a detectable extent. Additional research should be performed on eggs of other species and in other light environments, with an eye to an eventual generalized model of the effect of light on eggshell color.
Highlights
In 1897, experimental evidence corroborated the impressions of bird egg collectors and museum curators that the colors of bird eggs ‘‘are not very fast to light’’ [1]
Exposure was thoroughly overshadowed in the subsequent century’s research on egg color, which was dominated by the discoveries that it is consistent within an individual bird and has a strong genetic basis
Punnett’s [2] breeding experiments demonstrated that egg color differences in Chilean fowl (Gallus gallus Araucana) are genetically regulated, and that blue egg color is dominant to non-blue and can combine with gradients of brown through breeding to create a variety of egg colors
Summary
In 1897, experimental evidence corroborated the impressions of bird egg collectors and museum curators that the colors of bird eggs ‘‘are not very fast to light’’ [1]. Punnett’s [2] breeding experiments demonstrated that egg color differences in Chilean fowl (Gallus gallus Araucana) are genetically regulated, and that blue egg color is dominant to non-blue and can combine with gradients of brown through breeding to create a variety of egg colors. Recent research on the genetics of blueness in chicken eggs has culminated in the discovery of a retroviral insertion (EAV-HP) that strongly influences this trait [7] Concordant with these and other studies of the genetic bases for bird egg color differences, egg color has generally been shown to be robust to environmental variation [6, 8,9,10,11,12], licensing claims of evolutionary change from population comparisons [13]. In all of these cases, the changes in egg color are considered to be modulated by the physiology of the mother, and occur before the egg is laid
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