How bird eggs get their bling

Washington: Tinamou species are bird relatives of ostriches, rheas and emus.Pigments covered by a thin, smooth cuticle reveal the mystery behind these curious shells.

The team found the presence of a weak iridescence in the Tinamou's egg shells that creates superficial colours.

The colour of these eggs changes depending on the angle at which they are viewed and the light cast upon them.

"The perceived colour changes in relationship with the angles of observation and illumination. This effect can only be produced by nanostructures that influence how light is reflected," said Brani Igic from University of Akron in the US

By studying the mechanical construction behind the egg shell gloss and colouration, the team found that the Tinamou eggs' sheen results from an ultra-smooth coating, or cuticle, distinct from typically bumpy eggshells.

After removing the outer layer of the egg shells, the researchers examined their chemistry and nanostructure.

"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," added Matthew Shawkey, associate professor of biology at the University of Auckland.

"The research uncovers the longstanding mystery about the cause of these eggs' glossy appearance and shows that birds can make surfaces that rival those of highly-polished man-made materials," Shawkey said.

The findings appeared in the Journal of Royal Society Interface.