Bird eggs come in a host of beautiful colors and patterns, including the coffee-colored swirls of prinia eggs, the deep green of emu eggs, and the brilliant blue of the eggs laid by the humble robin. Scientists have long believed that this variation in eggshell appearance evolved relatively recently in modern birds, because crocodiles, our feathered friends’ closest living relatives, lay eggs that are completely white. But as James Gorman of the New York Times reports, a new study has found that colorful eggs may have appeared all the way back in the age of the dinosaurs.
In spite of their diversity, bird eggs derive their color from just two pigments: protoporphyrin, which produces a reddish-brown color, and biliverdin, which creates blue and green. Last year, a team led by Yale palaeontologist Jasmina Wiemann discovered these two pigments in the fossilized eggs of an oviraptor, a small, bird-like dinosaur. As part of a new study, published recently in the journal Nature, Wiemann and her colleagues expanded their research to include eggshells from 15 Cretaceous dinosaurs and extinct birds, along with the eggs of living birds like chickens, emus and terns, reports John Pickrell of Science.
The researchers relied on a non-destructive technique called Raman microspectroscopy, which involved bouncing lasers off the eggshells to map their molecular composition. The team detected the pigments protoporphyrin IX and biliverdin5 in the eggs of both modern birds and eumaniraptoran dinosaurs, among them the Velociraptor, which are ancient ancestors of today’s birds. Not only that, the researchers could see that some eumaniraptoran eggs were spotted and speckled. The pigments were even found at the same depths from the shell’s surface as the eggs of modern birds.
“We have, very likely, a single evolutionary origin of egg color,” Wiemann tells NPR’s Nell Greenfieldboyce.
More specifically, Wiemann thinks egg color evolved when certain dinosaurs started building open nests, rather than burying their eggs. Colors and patterns may have helped camouflage eggs that were newly exposed to predators, or allowed parents to recognize their soon-to-be hatched offspring, as is believed to be the case with modern bird eggs. In fact, birds that nest in enclosed spaces, like owls and woodpeckers, tend to have white eggs. Certain dinosaur eggs analyzed in the study, like those of certain sauropods and the duck-billed dinosaur Maiasaura, were not found to have any pigment, which may suggest that these species continued to bury their eggs in the ground.
The new study suggests there is good reason to rethink long-standing ideas about egg evolution, but the results of the research are not entirely surprising. Birds, after all, have inherited other traits from their ancient ancestors.