The Unanswerable Question

By Amy Renfranz (dearnaturalist@gmail.com)



Article Published: Mar. 20, 2013 | Modified: Mar. 20, 2013
The Unanswerable Question

American robins lay the most beautiful blue eggs. But why?

Photo submitted



Well, spring has finally sprung.

What lies ahead? Longer days, warmer temperatures and the birth or rebirth of our local flora and fauna.

What are you looking forward to, dear reader?

Do you daydream about wearing shorts? Do you ponder on fields of wildflowers, happy and well-informed tourists, and black bear campfire programs? Or is that just me?

One lucky reader looks forward to watching the nesting robins on her back porch. Every year, they arrive at this time of year, and the mother lays the first set of eggs.

Why are robins’ eggs blue? Do any other birds have blue eggs? –Nancy S., Boone

Your sweet momma robin will lay one egg every day until she has a brood of three to five. As the egg passes through her oviduct, it is essentially painted by pigments from ruptured hemoglobin in her blood.

Eggs from a robin will always be blue, and there are several possibilities for why this characteristic has evolved. One is that the blue is harder for predators to see than, say, a white colored egg. Another possibility is that with a brood of blue eggs, it is easier for the mother to spot an egg that does not belong.

There are some birds that will lay their eggs in another female’s nest, leaving her with the obligations of motherhood. These birds are known as brood parasites, and examples are the American coot and cowbird.

Yet, the white and brown egg of the cowbird would stand out like a sore thumb amongst robin eggs. The theory is that the mother robin could identify this intruder and push the bad egg out of her nest. This theory is, however, still being tested.

It is very hard for humans to make general assumptions on why birds do what they do, because our vision is very different from theirs. Birds have twice the number of color receptors in their eyes. Because of this, they see twice as many colors as we do, including ultraviolet.

What we see as a blue egg might appear different to the robin. How she uses color is still somewhat a mystery to science.

In other words, you have successfully stumped the naturalist. There is currently no definite answer on why a robin’s eggs are blue.

I do know, however, that there are several other birds that lay blue eggs. Grey catbirds, blue birds, European starlings and great blue herons all lay eggs of the cerulean variety. If you are really curious, look up the cassowary bird in Australia. This close relative of the dinosaurs lays eggs that vary in color from emerald green to indigo. Her eggs would put any Easter dye-job to shame!

If you have a question concerning flora and fauna, please email (dearnaturalist@gmail.com) All of your questions will be answered. One or two will be featured next week. See you on the trails!



Amy Renfranz is an interpretive park guide on the Blue Ridge Parkway. She is a certified naturalist through the Yellowstone Institute and a certified environmental educator in the state of North Carolina. Her comments are made independently and do not reflect the views of the National Park Service.


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