Used to be raptor egg blue (Image: Tzu-Ruei Yang, University Bonn)

The American robin lent its name to a striking shade of blue, but the vivid hue may have been colouring eggs long before the bird evolved – perhaps long before any birds evolved. It may have appeared in the dinosaur ancestors of birds that lived 150 million years ago.

Although recent studies have revealed the colours of dinosaur feathers, skin and scales, we had known nothing about the original colour of their eggs.

Ornithologists once assumed early birds, and the dinosaurs they evolved from, laid white eggs. But we know that some of the most ancient groups of birds still around – including the tinamou and emu – actually lay coloured eggs, points out Mark Hauber, an animal behaviourist at Hunter College in New York.


His group has discovered the chemical origin of the avocado-green from emu’s eggs, as well as the blue of robin’s eggs, the brown of chicken’s eggs and the pinks and purples from the eggs of other birds belonging to ancient living groups. The colours come from the way that two pigments in the shell – biliverdin and protoporphyrin – blend with each other and with the calcium carbonate that makes most of the shell.

True colours

But when were the two pigments first added to egg shells? Martin Sander of Bonn University in Germany has an idea. In a separate study, he looked at eggs from three prehistoric nesting sites in China where oviraptor dinosaurs – close relatives of modern birds – gathered to lay their eggs millions of years ago.

Sander chose the site because the eggs there are very pale. Dinosaur eggs found elsewhere are typically deep brown or black, because minerals have seeped into them over time and stained them, obscuring pigment molecules in the shell. The pigment molecules were more likely to be observed in the pale eggs from China – although in a fossilised state they would no longer colour the eggs in the same way they did when the egg was fresh..

Sander’s student, Jasmina Wiemann, found the oviraptor eggs contained both biliverdin and protoporphyrin. Most protoporphyrin came from the protein layer or cuticle still coating the fossil egg, as it does in modern bird eggs. Biliverdin came mostly from the calcium carbonate, also as in modern birds. Collectively the pigment evidence suggests oviraptors had blueish-green eggs.

The blue ones are mine

The similarity to modern birds probably reflects similarity in lifestyle, says Sander. Like most modern birds, the Chinese dinosaurs incubated their eggs in open nests, rather than burying them in the ground the way turtles or crocodiles do. Coloured eggs are an advantage in that situation, because they are much less obvious to the eager eyes of predators than white eggs. That means a brooding parent can slip away from the nest occasionally to snatch a meal.

Egg colour has other benefits: it can help the parent recognise and eject eggs that another species might add to the nest surreptitiously – like cuckoos and cowbirds do today. The protoporphyrin also helps strengthen the shells.

“This is our first knowledge of anything about dinosaur egg colours,” says David Varricchio of Montana State University, who was not involved in either study. Others failed to find colours in the past, he says, but techniques for identifying biomolecules have improved greatly since then.

He echoes Sander’s views on the link between coloured eggs and nesting behaviour – suggesting that it was dinosaurs switching to open nests that ultimately led to robin egg blue.

Journal references: Hauber’s study: Biology Letters, DOI: 10.1098/rsbl.2015.0087; Sander’s study: PeerJ Preprint 1080v1