by Heather Nicholson of Scratch Cradle

Have you ever wondered what makes blue eggs blue or why a green egg layer might hatch from a blue or brown egg? Did you know that brown eggs are coated with a “paint” made from blood, and blue egg shells contain a chemical byproduct of bile? Read on to find out the science behind egg color.

A Rose-Combed Brown Leghorn pullet which lays white eggs

White Egg Shells

Egg shell is made almost entirely from calcium carbonate (CaCO 3 ) and about 5% is made up from other minerals such as calcium phosphate, magnesium carbonate, and various proteins. White egg shells have no brown pigment over the surface or blue pigment within the shell. However, sometimes even white-egg-layers can lay a slightly brown or tinted egg after a long period of non-laying. Egg color is often (not always) tied to chicken earlobecolor. Chickens which lay white eggs tend to have white earlobes, like the pullet above. Mediterranean breeds, such as the Leghorn, are bred to lay white eggs.

White Egg-Laying Breeds: Leghorn, Ancona, Hamburg, Polish, Silkie, Icelandic, many others

Brown, white, green, and pink eggs

Brown Egg Shells

In contrast, Continental (other areas of Europe) and American breeds tend to have red earlobes and lay tinted (off-white or tan) eggs or brown eggs. Controlled by 13 or more genes, the varying levels of brown pigmentation seen on tinted and brown eggs is actually a coating applied during the last few hours in the hen’s uterus and isthmus. (You can see a diagram and read an article about the hen’s reproductive system here.)

This brown pigment is protoporphyrin, which is derived from hemoglobin or blood. A little brown pigment over a white egg will produce a slightly tan egg; a lot of this pigment will produce the deep chocolate brown seen in Marans eggs. Because this pigment is not part of the shell but more like paint on the surface, it can actually be washed off. It can also be scratched, and some nesting materials, like wood chips, can mar the surface of a brown egg when the “paint” is still wet. Because it is controlled by so many genes, brown egg color does not always act in a consistent way. Generally, if you cross a light egg layer with a dark egg layer, the resulting generation will lay an intermediate color. However, some breeds have a gene which can inhibit (lessen or block entirely) the expression of brown pigment.

Eggs are generally at their darkest in the fall and winter when hens lay fewer eggs. In late spring and summer, they are laying more heavily and more of their “paint” is used up. Older hens will also create less pigment; yet, their genetic material remains unchanged, and they can pass the genetics for the brown pigment they had in their youth onto their daughters (if the cock’s genetics support this as well.)

Brown Egg-Laying Breeds: Basque Hen, New Hampshire, Australorp, Cochin, Wyandotte, Plymouth Rock, many others

Dark Brown Egg-Laying Breeds:Marans, Barnevelder, Welsummer, Penedesenca



Blue Egg Shells (O)

Some birds from South America, such as the Araucana, carry a dominant gene for blue egg shells. In addition to calcium carbonate and the other typical minerals, blue egg shells contain oocyanin (thus the genetic symbol O; note the root “cyan”) which is a byproduct from the body’s production of bile. This blue pigment is not a coating like brown but is throughout the egg’s shell. Thus, the blue egg is also blue inside (before the inner membranes dry to a papery white) and is more difficult to see into when candling. However, like brown pigment, the amount of oocyanin will also lessen throughout a hen’s laying cycle, and she will lay progressively lighter blue eggs.

Interestingly, the gene for blue egg shells is closely related to the gene for a pea comb. A pea comb is a good predictor of the presence of the blue egg shell gene, but a chicken can have a pea comb and have white-shelled eggs. There is now a theory that blue-green pigments in egg shells help to protect the growing embryo from solar radiation. This may be why these pigments are often found in more breeds and species from tropical regions.

Blue Egg-Laying Breeds: Araucana, Ameraucana, Cream Legbar, some Easter Eggers

Green and Olive Eggs

When a brown coating overlays a blue egg shell, the egg appears green. If the brown coating is very dark, the egg appears olive. Today, many breeders are creating their own strain of mixed breeds to produce “olive eggers.” Generally, this is a cross between an Ameraucana and a Marans. To darken the olive tone, breeders often breed back to a Marans several times before stabilizing their line.

Green Egg-Laying Breeds: Isbar, Easter Egger (mutt), Olive Egger (mutt), rare South American breeds

Pink, Plum, Purple, and Gray

Current research acknowledges only two base shell colors: white and blue. However, pink eggs are common among Easter Eggers, purple or lavender eggs sometimes result from barnyard crosses, and Croad Langshans lay plum colored eggs. While it may be that a pink pigment can permeate the shell (see this forum discussion), pink coloring usually results from the bloom or cuticle which is a protein-like mucous coating applied by the hen’s reproductive tract just before the egg is laid which protects the egg from bacteria. This is why it is best not to wash clean eggs; washing removes the protective cuticle.

This cuticle, when applied over a white egg shell, creates a pink-colored egg. Just as thicker layers of protoporphyrin result in a darker brown egg, thicker layers of bloom result is a darker plum-colored egg as in that of the Croad Langshan (a breed which was actually used in the development of the Marans.) Gray appears to be the result of a thick layer of bloom over a blue or green egg.

Want to learn more?

If you are interested in learning more about egg color inheritance and chicken genetics, check out the Chicken Genetics Mini-Series on Scratch Cradle.