The wide range of colors found in wild bird eggs come from just two pigments – one brown and one blue – that also were used by birds’ dinosaur ancestors. The transition from plain to colored and speckled eggs likely occurred alongside the evolutionary shift away from covered, protected nests to the cupped, open nests more commonly used by birds today. In these exposed nests, shell color camouflages eggs from predators, aids in thermoregulation, and offers protection from ultraviolet light.
Researchers have found that eggshell color for some species is influenced by the presence of brood parasites which lay their eggs in the nests of others. Parasite parents don’t stick around to incubate the egg or raise young, leaving those tasks for the host species. A host that can identify and reject the parasite’s egg can focus its chick-rearing efforts on its own offspring. The result is an evolutionary color war with host species producing distinctive eggshell coloration which parasites then mimic. Recent research has looked at coloration of Australian Magpie eggshells in locations with parasite species, as well as the composition and surface properties of parasite eggs.