You can’t trust a bird’s egg color to know what’s inside

Birds produce some of the most colorful natural products: feathers and eggs. Surprisingly, the vast diversity of these colors is generated by just a handful of pigments and our HFSP Young Investigator team has worked on understanding how chemical simplicity in birds’ eggs generates its colorful appearance. We discovered that variation in the physical appearance of the eggshell color is not a reliable predictor of which pigments and in what relative concentration are incorporated into the eggshell itself.

HFSP Young Investigator Grant holders Phillip Cassey, Tomas Grim and Mark Hauber and colleagues
authored on Tue, 18 December 2012

The bright and colorful shells of bird eggs have long fascinated biologists and laypeople alike, because of the inherent duality of their function: to provide a stable environment for the embryos to develop inside the shell and to protect the egg from being detected by predators. Why, then, are many bird eggs so conspicuous? Our original HFSP Young Investigator team (Tomas Grim, Phill Cassey, and Mark Hauber) has been investigating this counterintuitive question using physical and chemical analyses of different bird species’ eggshells, collected from museums or from the field. Field work concentrated on two introduced species of thrushes: the European Blackbird and the Song Thrush, in New Zealand, so as not to impact negatively protected, endemic bird species.

Figure: Song Thrush lay blue eggs with sparse black spotting. Variation in the appearance of the eggshell color, however, is not closely related to the pigment concentrations deposited into these same shells. Photo credit: Tomas Grim. 

By analyzing the physical appearance of the eggshells, the chemical composition of the calcium-based shell material, and the content of the egg’s yolk, we focused on the possibility that variation in eggshell color within and between each nest serves as a visual signal to inform adult birds, including the parents, about the content of each of the eggs. We found no support for such a signaling hypothesis, and discovered that egg appearance and shell and yolk chemistry have only few, statistically weak links between each other. This implies that biologists must not assume that, by measuring the physical traits of the eggs, they will also gain information about what makes up the rest of the eggshell and the embryo’s environment.

Given that the bird's eggshell colors are generated by only two key pigments, biliverdin and protoporphyrin, these results have also provided the starting point for our HFSP Young Investigator Grant Renewal team (Mark Hauber, Tomas Grim, Matthew Shawkey, and Geoffrey Waterhouse), to begin exploring other non-chemical, including optical and nano-structural physical mechanisms, which may contribute, alter, and evolve to generate the diversity of egg colors across the ~10,000 known species of birds in nature, and the many varieties of domesticated stocks.

Reference

Avian eggshell pigments are not consistently correlated with colour measurements or egg constituents in two Turdus thrushes. Phillip Cassey, Ivan Mikšík, Steven J. Portugal, Golo Maurer, John G. Ewen, Erica Zarate, Mary A. Sewell, Filiz Karadas, Tomáš Grim, and Mark E. Hauber. 2012. Journal of Avian Biology 43: 503–512. doi: 10.1111/j.1600-048X.2012.05576.x

Journal of Avian Biology link