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Spring 2012

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

American robin egg, Turdus migratorius

May 1, 2012

Robin eggs: The bluer the better


Not much point in looking around for a nearby nest when you find an American robin eggshell on the sidewalk.

Soon after a chick hatches, the female robin grabs the eggshell and flies off to drop it far from the nest. Leaving the baby behind for a few moments is worth the risk, since the bright white insides of the eggshell can attract predators.

But before the egg hatches, blue-green pigments on the outside surface of the egg might help with camouflage. Pigments might also strengthen the egg and help protect it from solar radiation.

A robin coats her eggs with the same turquoise-hued compound found in our bile and bruises, biliverdin, an important antioxidant. Female robins with higher concentrations of biliverdin in their tissue lay darker, more vividly colored eggs, which apparently sends a strong signal to males.

"Males seem to use egg color to gauge the quality of their mate and the eggs she lays, putting more effort into rearing babies when they are more likely to survive and prosper," says Robert Montgomerie of Queen's University in Canada.

With Philina English, Montgomerie determined that when eggs are more colorful, male robins will invest as much as twice the amount of energy helping feed nestlings.

SOURCES:Behavioral Ecology and Sociobiology; Bird Coloration: Function and Evolution; Stanford University