Jay Margolis: Green eggs, blue eggs

Jay Margolis running a museum event booth with bird taxidermy and a duck-billed dinosaur display in the background. Birds are the only living group of dinosaurs, so evolutionary bird research often overlaps with paleontology. Photo courtesy of Lindsay Kastroll.

The following story was written by Jay Margolis as part of Dana Hawley's Outreach in Biology course.

Have you ever wondered why chicken eggs are usually brown or white, but robin eggs are bright blue? Me too!

Birds come in all sorts of colors, from brilliant blue jays to red northern cardinals, but why are their eggs different colors too? And how do birds make their eggs these colors?

My research as a second-year Ph.D. student in the Department of Biological Sciences is on the evolution of bird feather coloration and how bird color may be affected by behavior and the environment. While it is not the primary focus of my dissertation research, I've also been curious about influences on bird egg color. There is already a lot of fascinating, but dense, literature that exists on egg coloration that I have read, and I’d like to share what I’ve learned.

One of the main reasons birds, and their eggs, are certain colors is to blend into the background of their environment. Coloration like this that allows an animal to blend into its environment is called cryptic coloration, and this allows animals to avoid predators or avoid being seen by their prey (Wallace, 1891). For instance, in many bird species the female bird is less bright or colorful than the male is. In these cases, such as in many species of warblers and finches, the female is sitting on the nest the most, and the lack of bright colors allows her to be more camouflaged and prevents the nest from being noticed by predators (Martin & Badyaev, 1996).

Egg specimens from the Carnegie Museum of Natural History in Pittsburgh, PA. To preserve eggshells for museums small holes are made to drain the inside of the egg, keeping the remaining shell intact. One notable egg pictured is an off-white Wood Duck egg from 1863. Photo courtesy of Jay Margolis. — Egg specimens from the Carnegie Museum of Natural History in Pittsburgh, PA. To preserve eggshells for museums small holes are made to drain the inside of the egg, keeping the remaining shell intact. One notable egg pictured is an off-white wood duck egg from 1863. Photo courtesy of Jay Margolis.

Like adult bird coloration, some bird species have eggs that are colored in ways that can help them be less visible to predators. Many species nest on the ground, which makes their nests more available for predation and makes camouflaged eggs beneficial. Take the eggs of the semipalmated plover, for example. A 2010 experiment showed through digital photography and nest observation that natural plover nests with cryptic eggs were more successful at avoiding predation than artificial nests with eggs that did not blend into the surrounding environment (Nguyen, Nol, & Abraham, 2010).

So, we know that some eggs are colored in ways that help them blend into the surrounding environment of the nests they’re in, but what about those bright blue robin's eggs?

It turns out that blue color may be a way for a parent bird to protect its growing offspring from the sun. If you’ve been to a beach as a child, you likely had the experience of a doting parent insisting you wear sunscreen to protect you from the sun’s harmful rays. All animals — not just humans — must contend with protecting themselves from ultraviolet, or UV, radiation that comes from the sun, as it can damage DNA and cause cancer, such as skin cancer in humans (Leffell & Brash, 1996).

An Eastern Bluebird nest: Four sky-blue eggs and a pink featherless hatchling. — An eastern bluebird nest: Four sky-blue eggs and a pink featherless hatchling. Photo courtesy of Jay Margolis.

Bird parents don’t have a bottle of sunscreen to protect their eggs from the sun’s UV radiation, but they do have the ability to put colored pigments into their eggshells. How much, and which colors, is very important — because radiation from the sun can cause two forms of harm.

Researchers David Lahti and Daniel Ardia, in a 2016 publication with the great title “Shedding light on bird egg color: Pigment as parasol and the dark car effect,” explained that birds use the amount of pigment in their eggs to balance a tradeoff between two forms of harm from the sun that can damage the developing embryos inside eggs. These scientists categorized the two forms of harm as 1. transmittance, or solar rays coming through the eggshell, which can expose the embryo to harmful UV radiation, and 2. absorbance, heat building up inside the egg, which can overheat the embryo (Lahti & Ardia, 2016). Too much color in the eggshell can result in too much absorbance, like wearing black clothing on a sunny day, but too little color means the eggshell does not block enough UV radiation.

But what causes the blue or green colors? The blue color birds use in their eggs can come from two different color pigments, oocyanin for cyan, a deep blue, and biliverdin for blue. Zinc chelate is the pigment used by birds for greens (Burley & Vadehra, 1989). These colors get incorporated into the bird’s eggshells during the egg-laying process, where some birds even have the ability to control the amount of color that gets deposited.

Next time you see eggs, whether you’re eating them or you have the fortune to see a wild bird nest, I hope you’ll think about all the cool aspects of eggshell color. And maybe it’ll remind you to reapply your sunscreen!

Works cited:

Burley, R.W. & Vadehra, D.V. (1989), The avian egg. Chemistry and Biology. New York; John Wiley and Sons.

Lahti, D. C. & Ardia, D. R. (2016), Shedding light on bird egg color: Pigment as parasol and the dark car effect. American Naturalist, 187, 547–563. https://doi.org/10.1086/685780

Leffell, D. J., & Brash, D. E. (1996). Sunlight and skin cancer. Scientific American, 275(1), 52–59. http://www.jstor.org/stable/24993269

Martin, T. E., Badyaev, A. V. (1996). Sexual dichromatism in birds: importance of nest predation and nest location for females versus males, Evolution, 50: 6, pp. 2454–2460, https://doi.org/10.1111/j.1558-5646.1996.tb03631.x

Nguyen, L.P., Nol, E., & Abraham, K.F. (2007), Using Digital Photographs to Evaluate the Effectiveness of Plover Egg Crypsis. The Journal of Wildlife Management, 71: 2084-2089. https://doi.org/10.2193/2006-471

Wallace, A. R. (1891), Natural selection and tropical nature; essays on descriptive and theoretical biology. “New edition with corrections and additions." London & New York: Macmillan & Co.