Shaped like tiny ostriches (but without the gams), tinamous are well-camouflaged, drab-colored birds that lay some of the most eye-catching eggs: Their shells glisten with the luminous sheen of a nicely buffed mirror. And that’s not it. The females of one tinamou species in particular lay eggs that change color depending on what angle you view them from. This is the first time researchers have documented natural iridescence in an avian eggshell. The findings were published in the Journal of the Royal Society Interface this week. 

Despite the colorful variety of bird eggs found in nature, just two pigments contribute to the majority of their visual appearance. Tinamous however, have an additional trick: They lay color-changing eggs with a mirror-like gloss. 

To understand the mechanism responsible for these optical effects, a team led by Branislav Igic from the University of Akron obtained unincubated eggs from four tinamou species (order Tinamiformes): blue eggs of the great tinamou (a, below), green eggs from the elegant crested tinamou (b), brown eggs from the Chilean tinamou, and dark brown eggs from the spotted nothura (c). For comparison, they also examined the bluish, matte eggs of the Araucana chicken. 

Using a bit of pressure, the team fragmented the eggshells, and then they measured gloss and iridescence, conducted scanning electron microscopy (SEM) of the surface topography, and examined the protective cuticle layer using chemical analyses. That mirror-finish glossiness, they found, is produced by a thin and extremely smooth cuticle made of calcium carbonate, calcium phosphate, and a probable mix of organic compounds like proteins. 

Furthermore, the team also documented the weak presence of natural iridescence on the eggs of the great tinamou (Tinamus major), also known as the mountain hen. The color that we perceive changes depending on the angle of observation and illumination. Until now, this optical effect has never been documented for bird eggs. The findings highlight the role of nanostructures in modulating the appearance of eggshells—color isn’t all about pigments.

But why would such an unassuming bird lay such vibrant eggs that are all the better for predators to see? The researchers suspect it has to do with the tinamou mating system. Multiple females lay their eggs in the same nest, which is then incubated by a single male. Because gloss and color fade in the incubation process, these may be cues used by females to assess the age of nests—and to avoid laying eggs where incubation has already begun. Bright eggs might also serve to blackmail the males, forcing them to be more attentive when concealing the conspicuous eggs from predators. Compared with other birds, male tinamous do have extraordinarily high incubation attendance rates. Alternatively, gloss and iridescence might just be the byproduct of the mechanisms that protect developing embryos. Surface smoothness prevents water from clogging pores and impending gas exchange, while highly reflective surfaces may help prevent sun damage. 

Images: Brian Gratwicke via Flickr CC BY 2.0 (top), Karsten Thomsen, Sam Houston and Shirley Sekarajasingham from 2014 B. Igic et al., Royal Society (middle)