From left to right: Eggs of the great tinamou (Tinamus major), elegant crested tinamou (Eudromia elegans), and spotted nothura (Nothura maculosa). Photograph of great tinamou egg (UMMZ 191600) used with permission from the University of Michigan Museum of Zoology. Photo credits: D. Hanley (great tinamou) and M. Hauber (elegant crested tinamou and spotted nothura).

Relatives of ostriches and emus, Tinamou birds, lay supershiny eggs, but until now no-one knew how the eggs got so beautiful.

Researchers at the University of Akron in the USA looked carefully at the eggs and found they were shiny thanks to a thin. smooth outer covering over the shell.

“This smoothness causes light to be reflected in a specular manner, like off of a lake or mirror. The bumpiness of other eggs causes them reflect light diffusely, like a cloud,” Matthew Shawkey, associate professor of biology, explains.

If the researchers can find out how the birds make the gloss, it might be possible to develop new coatings for ceramics and floors.

The eggs are also iridescent, meaning the colour they appear depends on the angle you look at them from.  Other iridescent surfaces are butterfly wings or some seashells.

The researchers removed the outer layer and found the egg surface beneath had teeny tiny structures that made the iridescence.