If you have seen a nest with bright blue eggs, you may be wondering what the unique pigmentation does.

Now, researchers in a new study have discovered that the color could shield harmful sunlight, particularly ultraviolet radiation.

David Lahti from the City University of New York and Dan Ardia out of Franklin and Marshall College have tested their hypothesis: pigmentation might help birds’ eggs balance out two opposite effects of the sun, namely transmission of light onto light-hued eggs and heating up of dark-colored ones.

“We quantitatively test four components of this hypothesis on variably colored eggs of the village weaverbird in a controlled light environment,” wrote the authors.

The theory proved true during the experiments: a light blue tint allowed the eggs to be warm without overheating. The more intensely colored blue eggshells better protected the insides from potentially from harmful UV radiation.

However, the color also led eggs to absorb more light and create an internal temperature that could be too hot for the baby’s incubation.

Dark-tinted eggs may be particularly dangerous in tropical environments, where the sun shines bright and is barely shielded by clouds. Excessively light-colored eggs, on the other hand, may usher in too much UV radiation and not provide enough heating.

Dubbed as “pigment as parasol” and “dark car effect,” the two resulting patterns can be paired with the knowledge of the birds’ habitats and nesting behavior in order to carefully predict why birds’ eggs vary across species in color.

These findings show that if camouflage to protect from predators is the primary factor behind the evolution of dull, mottle egg colors, the varying effects of the sun are the answer behind brighter egg colors.

The findings were published in the journal The American Naturalist.

Birds’ eggs play a crucial role not only in nature, but also in captive breeding techniques. A strategy called “double-clutching,” for instance, involves the snatching of eggs from nests and hatching them under incubators, promoting mother birds to lay another set of eggs in the wild.

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