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Planet Earth

Flashback Friday: Why Do Some Birds Lay Blue Eggs?

New study shows that blue eggs absorb just the right amount of heat for the baby birds inside.

Robin's Eggs - Shutterstock
(Credit: Brandon Blinkenberg/Shutterstock)


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While you might be most familiar with "robin's egg blue", many species of birds lay blue-colored eggs. Why might this have evolved? Although scientists can't go back in time to observe the emergence of blue eggs, they can think carefully about which properties might be most different between blue and non-blue eggs — which is what these researchers did!

They found that blue eggs absorb just the right amount of light to warm the egg, but not allow it to get too hot. Egg-cellent!

Shedding Light on Bird Egg Color: Pigment as Parasol and the Dark Car Effect

"The vibrant colors of many birds' eggs, particularly those that are blue to blue-green, are extraordinary in that they are striking traits present in hundreds of species that have nevertheless eluded evolutionary functional explanation.

We propose that 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.

We quantitatively test four components of this hypothesis on variably colored eggs of the village weaverbird (Ploceus cucullatus) in a controlled light environment:

(1) damaging ultraviolet radiation can transmit through bird eggshells,

(2) infrared radiation at natural intensities can heat the interior of eggs,

(3) more intense egg coloration decreases light transmittance ("pigment as parasol"), and

(4) more intense egg coloration increases absorbance of light by the eggshell and heats the egg interior ("dark car effect").

Results support all of these predictions. Thus, in sunlit nesting environments, less pigmentation will increase the detrimental effect of transmittance, but more pigmentation will increase the detrimental effect of absorbance.

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."

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