Why Bird Eggs Shimmer and Shine with Glossy Hues

Some bird eggs are colored in vibrant hues, ranging from emerald green to blue to yellow. Now, scientists have taken a closer look at the pigments within these shells and have found out a bit more about shell coatings.

Egg shells are known for their sheen-the thin, smooth cuticle that covers the pigments. In order to learn a bit more about these shells, the researchers removed the outer layer of eggshells and examined their chemistry and nanostructure. Surprisingly, the scientists found a weak iridescence in the Tinamou's eggshells, which created superficial colors and color changes depending on the angle at which they're viewed. Tinamou species, in particular, are bird relatives of ostriches, rheas and emus.

"The perceived color changes in relationship with the angles of observation and illumination," said Brani Igic, one of the researchers, in a news release. "This effect can only be produced by nanostructures that influence how light is reflected."

More specifically, the scientists found that the iridescence from the egg is due to an ultra-smooth cuticle that's distinct from typically bumpy eggshells. This smoothness causes light to be reflected in a specular manner-the way in which light is reflected off of a lake or a mirror.

"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 humanmade materials," said Matthew Shawkey, one of the researchers. "How they do this during egg development and why are great questions for the future."

The findings could potentially help researchers create materials based on these shells. For example, it's possible to develop glossy new coatings for ceramics and floors, potentially enhancing their aesthetic qualities and durability.

The findings are published in the Journal of the Royal Society Interface.

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