Biology's Blue Egg Mystery Explained

A Franklin & Marshall College biologist and a colleague at the City University of New York have answered the age-old question, why are robin eggs blue?

F&M Associate Professor of Biology Daniel Ardia and CUNY Assistant Professor of Biology David Lahti determined the color protects the bird embryo from the harmful rays of the sun.

"Egg pigmentation mediates a trade-off between two routes by which solar radiation can harm bird embryos: transmittance through the eggshell and overheating through absorbance," the researchers reported.

In their paper, "Shedding Light on Bird Egg Color: Pigment as Parasol and the Dark Car Effect," which first appeared in The American Naturalist, Ardia and Lahti substantiated their theory by examining four environmental conditions on variably colored eggs of the village weaver bird, a species found in sub-Saharan Africa.

Working in a controlled light environment, they found ultraviolet radiation can transmit through bird eggshells; infrared radiation at natural intensities can heat the interior of eggs; more intense egg coloration (hence the robin's deep blue egg) decreases light transmittance – the “pigment as parasol” theory – and more intense egg coloration increases absorbance of light by the eggshell and heats the egg interior – the “dark car effect."

According to the researchers, the darker colored eggshells did a better job at protecting from damaging UV rays, but caused the embryos to heat faster.

"In sunlit nesting environments, less pigmentation will increase the detrimental effect of transmittance, but more pigmentation will increase the detrimental effect of absorbance," the researchers wrote. "The optimal pigmentation level for a bird egg in a given light environment, all other things being equal, will depend on the balance between light transmittance and absorbance in relation to embryo fitness."