In the current issue of Ecology Letters, Moreno and Osorno interpret vibrant egg colours as sexually selected signals of the laying females' genetic quality to their mates in order to induce them to work harder for offspring that will inherit the advertised traits. According to modern theories of sexual selection, signals have to be costly to be reliable and evolve.
The blue-green pigment biliverdin is a potent antioxidant the deposition of which in eggshells may advertise the heritable capacity to destroy deleterious free radicals. Using antioxidants to colour their eggs may signal that females can sustain the cost of removing them from metabolism.
Egg ground colour has been shown to be heritable in some species and very variable within avian taxa, a typical characteristic of sexually selected traits. The hypothesis can be applied to all animals with colourful eggs and paternal care.