It turns out that a robin’s egg blue could predate the robin by millennia.
Animals that lay eggs – all birds, some reptiles and two species of monotreme mammal – are known an amniotes. Birds are the only ones which lay eggs that come in different colours – the result of an adaptation inherited from their dinosaur ancestors, says a new study in the journal Nature.
Researchers, led by Jasmina Wiemann at Yale University in Connecticut, US, use Raman spectroscopy to analyse fossil eggshells from all the major dinosaur groups.
In those of a group of small, bipedal and often feathered animals known as Maniraptora, the scientists found traces of pigment. None was found in eggs for any other type.
“Egg colour had a single evolutionary origin in nonavian theropod dinosaurs,” the authors conclude.
“As with many other characteristics, this is an attribute that evolved deep within the dinosaur tree and long before the spectacular radiation of modern birds.”
The finding runs counter to previous assumptions. Coloured eggs “have long been considered to be an avian innovation,” the researchers note.
Wiemann and colleagues suggest that pigmentation may have evolved in response to environmental cues, with colouring giving a selective advantage to eggs left uncovered or unburied.