Biologists Solve Mystery of Blue-Green Bird Eggs
Blue-green egg color shields bird embryos from harmful sunlight, according to Dr. David Lahti from the City University of New York and Dr. Dan Ardia from Franklin & Marshall College.
Blue-green eggs of the Eastern bluebird ( Sialia sialis ). Image credit: Eastern Kentucky University.
The authors tested the hypothesis that pigmentation might help an egg strike a balance between two opposing and potentially damaging effects of the Sun: light transmission into light-colored eggs, and heating up of dark-colored eggs.
“We quantitatively test four components of this hypothesis,” they wrote in a paper published in the May issue of The American Naturalist , “on variably colored eggs of the village weaverbird ( Ploceus cucullatus ) in a controlled light environment:
(i) damaging ultraviolet (UV) radiation can transmit through bird eggshells;
(ii) infrared (IR) radiation at natural intensities can heat the interior of eggs;
(iii) more intense egg coloration decreases light transmittance (pigment as parasol);
(iv) more intense egg coloration increases absorbance of light by the eggshell and heats the egg interior (dark car effect).”
As predicted, more intensely blue eggshells shielded the interior from light, including dangerous UV radiation.
But more intense color also caused eggs to absorb more light and heat up, which can be even more dangerous in brighter environments.
These patterns, combined with knowledge of the nesting behavior and habitats of birds, can lead to predictions as to why the eggs of some birds vary across species from blue to white.
Darker eggs are predicted in moderate light to shield the embryo, but in brighter nests the dangers of egg heating predict lighter colored eggs.
Whereas camouflage from predators is still probably the single most important factor governing the evolution of dull and mottled egg colors, for the brighter colors the biophysical evidence points to the Sun.