What Causes Chicken Eggs to be Blue

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There are a variety of chicken egg colors. But most of the time we only see white and brown. Though I’ve heard that some stores carry blue eggs. I sell blue (and brown) eggs to some neighbors and people I work with. However I also give a lot away. Recently I was asked, “What causes chicken eggs to be blue?” Do you ever get this question? Or have you asked it yourself?

I think people are under the impression, that because the shell is colored differently, somehow it will affect the taste. But that just isn’t the case at all. Although all farm fresh eggs taste differently compared to store bought eggs. That is if they’re free-range or pasture raised eggs. And that’s due entirely to the hen’s diet, not to the shell color.

broken chicken egg placed on white table
Photo by Klaus Nielsen on

Additionally, I think people get the idea that shell color determines how healthy the egg is. And once again, that isn’t true. But eggs from large farms, where the chickens are all caged, are going to taste different from the eggs my birds produce. And just why is that? To a large extent, that’s due to the bird’s freedom to forage. If a bird is caged all day, without interaction from her peers or social order, she’s basically just a machine. Or a tool. Eggs from a farm taste richer. And the yolks are more orange than yellow, signifying a healthier diet for the hens. Which will result in more omega-3s and vitamins for those who consume them.

So Why are Chicken Eggs Different Colors

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Photo by Markus Spiske on

Well, firstly, most egg colors are determined by the hen’s genetics. And all eggshells are made of calcium carbonate, including the white ones. They’re just lacking pigment. But also, all eggshells begin as white. Although eggs that have shells in colors other than white will have the pigment levied on them as they make their way through the hen’s oviduct. Kinda like the hen is painting the egg. Weird, right?

The pigment, known as protoporphyrin IX, is responsible for staining eggs brown. Which is deposited later in the whole process, only making the eggs brown on the outside. So, certain hens only lay brown eggs, while others only lay white. But where did the blue come from?

Well, blue eggs also have a gene that’s essential to their pigmentation called oocyan. The key differences between protoporphyrin IX and oocyan is that blue eggs are blue inside and out. And the gene oocyan got there because of a retrovirus, called EAV-HP, hundreds of years ago. This was learned in a study completed around 2013 by the University of Nottingham.

Cream Legbar hen in mixed flock
This is our Cream Legbar hen, just one of many of our hens that lays blue eggs.

Furthermore, one of the first birds to have acquired the blue egg trait, and thus the virus, were the Mapuche fowls of Chile, possibly 500 years ago. Consequently, these birds are ancestors to French, Spanish, North American, and British chickens, such as the Araucana, Ameraucana, and Cream Legbar chickens, to name a few. And Asia has their own breeds with the oocyan gene, the ancient Dongxiang and Lushi.

But the virus isn’t harmful. In the case of causing pigmentation in chicken eggs, it’s actually pretty cool. We’re all profoundly aware that viruses can cause sickness, including foodborne illnesses. However they can also change an animal just slightly, like EAV-HP did, resulting in blue eggshells.

I hope you enjoyed this post, and I thank you for taking the time to read it.


I breed pure Black Ameraucana chickens and Easter Eggers that are Black Ameraucana mixed with either Cuckoo Maran or Barred Rock. And I donate eggs to people or organizations in need.

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