Researchers at the University of Nottingham, UK and in China have recently reported on genetic findings that tell us that the blue colored chicken eggs highly favored by some food gourmands happen because of a retrovirus. The retrovirus that causes blue eggs is harmless and enters the DNA of chickens which causes the blue pigment biliverdin to be deposited on egg shells. The pigment naturally occurs in animal bodies, it causes liver bile to appear greenish among other things.
Interestingly this mutation took hold in two different places, South America and China, at two different times in domestic chickens. In China the mutation occurred roughly 500 years ago and in South America the mutation probably occurred 2-3 hundred years ago. Scientists know the mutation happened in two different places because the location where the altered DNA is found on the chicken genome is different in Asian and South American breeds. Humans then continued to select chickens and breed them to produce blue eggs.
The ancestors of domestic chickens, the Jungle Fowl, laid white eggs. In fact the original egg color for all birds was white. In some species of birds a mutation that caused brown pigmentation or brown spots developed. The brown pigments are protoporphyrins, (pigments that also help make blood red) and can be deposited in varying degrees on egg shells during development. Some birds also developed blue or green colored eggs when a mutation causes the pigment biliverdin to be deposited on egg shells. This is the mutation that was recently studied in domestic chickens. We don’t know if the retrovirus passed to chickens from other species of birds or developed independently.
How nature does it
Nature was the selective force for the brown color or spotting of eggs that some birds developed. If the bird nests on the ground or in an open spot eggs that blend into the surroundings have a better chance of survival. Soon some species of birds began to lay only brown or spotted eggs. Nature may have also selected for blue or green eggs in some species of birds. We don’t know why it helps but in wild birds in which both parents feed the young and are not cavity nesters, the eggs are more likely to be blue or greenish. Some species of birds, such as the house sparrow, lay either white or blue eggs, depending on the individual. Birds that lay their eggs in a cavity, where it is dark, tend to have white eggs, since being able to see the eggs makes more sense and is favored in natural selection.
In domesticated chickens the color of eggs was selected for by humans, especially when it comes to blue eggs. Brown chicken eggs are older than blue eggs, the mutation occurred early in domestication. Brown egg color is linked to larger body size in chickens to some extent and people may have been selecting for that and ignoring egg color. Another mutation occurred early in chicken domestication and that involves the chickens laying pattern in response to day length. Humans selected for chickens that laid through the winter when daylight was shorter. It’s possible that early brown layers were less responsive to day length and laid more eggs.
Chicken breeds that produce blue or green eggs include, Araucana/ Mapuche, Cream Legbar, Moss, and Ameraucanas and a catch-all called Easter Eggers are derived from the South American mutation and Dongxiang and Lushi, are Chinese mutation derived. Some of these breeds are quite rare now. There are hundreds of chicken breeds that lay brown eggs.
A chicken egg spends about 20 hours in the shell gland on its 25-26 hour trip through the hen’s reproductive system. Then, if there is pigment in the shell gland that pigment is deposited on the last few layers of shell that are deposited around the egg yolk and white. As a hen gets older egg color can lighten, usually this is because older hens lay larger eggs but the amount of pigment that each egg gets is the same. In some cases spots of brown pigment can actually be scraped off the outer layer of shell but blue pigment cannot.
Looks don’t mean much
White eggs became the standard for commercial eggs with the development of the white Leghorn and other production egg laying breeds. The birds were small and took less space and feed but still produced a good sized egg. In some places however, brown or blue eggs have always been favored.
Brown and blue colored eggs have exactly the same taste and nutrition as do white eggs. The eggs look the same inside. Eggs taste better when they are fresh and when the hens get to roam a bit and eat lots of greenery. Eggs from hens that eat greens also have deeper yellow yolks. It’s possible that people think brown and blue eggs taste better because these breeds are more likely to be raised in a farm setting than Leghorns which lay white eggs and that are kept in small cages.
Feather color by the way, has nothing to do with egg color. A white chicken can lay brown or blue eggs and a brown chicken can lay white or blue eggs. The color of a hen’s earlobes does give a clue to egg color, brown egg layers usually have red ear lobes, but it’s not reliable for all breeds of chickens.
When a chicken that lays blue eggs is crossed with a chicken that lays brown eggs the eggs are often an olive color because both the brown and blue pigment mutations affect the egg color. Green eggs are laid when a chicken with the blue mutation just expresses the color a bit differently or when the chicken has a light amount of brown pigment also present. When blue egg laying chickens are crossed with white egg laying chickens the eggs are usually lightly tinted or they appear white. The three colors white, brown, blue, can be combined in various ways to provide various tints. Really dark brown eggs are produced by breeding chickens with dark eggs and selecting for the darkest brown eggs over generations.
If you would like to read the scientific article that recently linked the retrovirus to blue egg color you can do so here.
Here are some additional articles you may want to read.
How to get young hens to begin laying eggs
Which eggs taste better
Keeping chickens humanely at home
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