The function and evolution of egg colour in birds

Animal colouration generally evolves via natural or sexual selection, or some combination of the two. From a naturalist‟s perspective, the diversity of colour exhibited by avian eggs is particularly interesting, because much of this diversity has not been thoroughly explained by either mode of selection. Until recently, a sexual selection mechanism for the evolution of egg colour was not known, and natural selection did not appear to be acting on some egg colours, most notably the unspotted white and blue-green eggs laid in open nests. The goal of my dissertation is to investigate the functional significance and selective pressures facing the evolution of egg colour. In Chapter 2, I investigate whether egg colour serves as signal of female quality. I find little support for this hypothesis and suggest that future research should examine other explanations for the evolution of egg colour. In Chapter 3, I find that environmental contaminants have a significant influence on egg colour. This has important implications for employing eggshell pigmentation as a non-destructive bio-indicator. In Chapters 4 and 5, I conduct large-scale comparative analyses that involve the reconstruction of a super-tree including representatives of all but one avian order. In Chapter 4, I find that predation is negatively related to ultraviolet chroma in open nests, and eggshell brightness is positively related to predation pressure in species using open nests above the ground. In addition, the risk of brood parasitism is greatest in species with a high proportion of blue-green chroma, but nest attendance is higher for these nests, suggesting that parents may behaviourally mitigate the risks of parasitism. I also find greater variation between clutches in species that experience high rates of parasitism; this presumably makes spotting a brood parasitic egg easier. In Chapter 5, I find that within cavity nests, selection is acting to increase eggshell brightness. I also find suggestive evidence that eggshell pigments could be adapted to protect the embryo from harmful solar radiation. In Chapter 6, I document and describe eggshell phosphorescence, a previously undocumented property, and suggest that this property is due to porphyrin within the eggshell.