When it comes to shell color, most birds’ eggs conform to one of four motifs: colored with spots, colored without spots, white with spots or pure white.

But there is a striking exception: the eggs of the tinamou, a shy, ground-dwelling bird native to South and Central America. Tinamous, which look something like a cross between a guinea hen and a roadrunner, blend in with their forest environment by sporting drab plumage in shades of brown and gray. But they produce some of the most spectacular eggshells in the world, in colors like sky blue, lime green and rich chocolate and so glossy that they can reflect overhead trees and brush.

Now a study in The Journal of the Royal Society Interface takes a close look at the tinamou eggs and finds that they possess a unique structure and iridescence — the only known example ever found in eggshells. “Working with these eggs was, for me, like becoming an artist,” said Mark E. Hauber, an animal behaviorist at Hunter College and author of The Book of Eggs. “They are so unique and unusual that it was hard to take my eyes off them.”

To crack the structural secrets of the eggs’ ceramic appearance, Dr. Hauber and his colleagues first obtained green, blue, light brown and dark brown eggs from four species of tinamou kept by the Bronx Zoo, the Dallas World Aquarium and a private breeder.