The rainbow of hues seen in modern bird eggs probably evolved in birds' dinosaur ancestors, which had eggs with colorful and speckled shells.
That's according to a new study of fossil eggs in the journal Nature. Researchers found that birds' close dinosaur relatives had eggs with traces of two pigments—a red-brown one and a blue-green one. This same pair of pigments mixes and matches in today's bird eggs to produce colors ranging from robin's egg blue to red to yellow to green.
"There is a huge diversity in egg color and pattern. For a long, long time people have assumed that egg color is a trait that is unique to our modern birds," says Jasmina Wiemann, a paleontologist at Yale University. She says that assumption was based on the fact that birds' closest living relative, the crocodiles, "have completely uncolored, unpigmented eggs."
To see if colored eggs might actually go further back in history, she and her colleagues started by looking at the eggs of oviraptors-- a relative of the velociraptor made famous in the movie Jurassic Park.
"This dinosaur is particularly interesting because oviraptors are the first dinosaurs that built open nests," she says, explaining that earlier dinosaurs buried eggs underground, where color wouldn't be expected to make any difference.