Tinamous have eggs with shells in bright, shimmering colors that can appear to change as you look at them. The shy, turkey-size South American birds are the only animal known to lay eggs that are iridescent. The cuticle, or outermost layer, of a tinamou egg has special characteristics at the nanostructural level that affect how it reflects light. This “structural coloration” plus pigment produces colorful, glossy eggshells. Why secretive tinamous need such Technicolor eggs remains a mystery. Multiple females lay their eggs in one nest, which is then incubated by a male, so perhaps this may have something to do with brightness.
Evolution's oddest eggs: http://goo.gl/2uxba5
The Great Tinamou Lays the Fanciest Eggs in the Bird World http://goo.gl/qvM3ko