The Science Behind Blue Eggs

Although most breeds of chickens lay white or brown eggs, there are a few breeds that lay blue ones. In the U.S., you may come across Araucana, Americauna, and Easter Egger chickens. Araucana chickens are descended from chickens in Chile called Mapuche fowl. They have tufts of feathers around their ears, and they are rumpless – they have no tail or oil gland (1).

Araucana hen with ear tufts by Anne Cushing

Araucana hen with ear tufts
by Anne Cushing

Ameraucana chickens were bred from Araucanas. These have muffs (extra feathering on the sides of the face) and beards (extra feathering under the chin), a pea comb, a full tail, and dark legs (2).

Blue Ameraucana Cock By Royale Photography

Blue Ameraucana Cock
By Royale Photography

Easter Eggers are any chicken with the blue egg gene, and typically look a bit like the Ameraucanas with the tail, beard, and muffs, but not always. Here is my Easter Egger girl, Sue.

Sue (Easter Egger) on driveway

Sue (Easter Egger) on driveway

Sue (Easter Egger) in the grass

Sue (Easter Egger) in the grass

Notice her pea comb, extra feathering around the face, her tail, and dark legs. She looks similar to Ameraucanas, which is pretty typical for an Easter Egger, but it’s not always the case. Unlike the Ameraucana, this is not a true breed, so if two Easter Eggers mate, the offspring won’t necessarily look like the parents.

Aside from the Araucanas, Ameraucanas, and Easter Eggers, the blue egg gene is also found in some Asian chicken breeds, such as the Dongxiang.

Scientists have known that the blue pigment in these eggshells is a bile pigment called biliverdin, which gets absorbed into the eggshell in the shell gland of the hen (4). But until recently, they didn’t know what gene caused that to happen. This year, scientists published about the genetic cause of blue eggs.

Normally in biology, the DNA is what encodes the blueprint for the organism. When a protein needs to be made, messengers called RNA are produced by transcribing from the DNA blueprint, and then the RNA is directly used for instructing the cells to make proteins. So the paradigm is DNA->RNA->protein.

The scientists discovered that blue chicken eggs are caused by an endogenous retrovirus. This means that instead of a virus using the normal paradigm of DNA->RNA->protein, there was an RNA virus that “reverse transcribed” itself into DNA. This DNA then inserted itself into the chicken genome. In two separate cases (the Mapuche fowl of Chile, and the Dongxiang of Asia), this insertion happened near a gene encoding a carrier protein for biliverdin. Because the insertion happened so close to the carrier protein gene, a lot more carrier protein gets produced in the shell gland of the chicken, causing biliverdin to be incorporated into the shells (3).

All this talk of viruses might sound scary, but don’t worry – your Easter Egger chickens aren’t sick! Retroviruses in the genomes of organisms are very, very common. For example, endogenous retroviral elements make up 8% of the human genome (5). But almost all of them have mutations making them inactive, so they can’t spread to new areas of the genome and cause problems. The blue egg retrovirus is one of those that have been inactivated by mutations (3).

Prior to this finding, retroviruses were always believed to insert randomly into genomes, but this shows an example of a retrovirus inserting twice into almost exactly the same place. Blue eggs are not uncommon among bird species.  Robins, house finches, bluebirds, and starlings are a few examples. It will be interesting to see if these species have similar retroviruses causing their blue eggs. And if they do, why are retroviral insertions into this one spot so common? I can see this having big implications for basic biology down the line!

American Robin Eggs in Nest by Laslovarga

American Robin Eggs in Nest
by Laslovarga




3. Wragg D, Mwarcharo J, Alcalde J, Wang C, Han J-L, Gongora J, Gourichon D, Tixier-Boichard M, Hanotte O (2013) Endogenous retrovirus EAV-HP linked to blue egg phenotype in Mapuche fowl. PLOS One 8:e71393.

4. Gorchein A, Lim CK, Cassey P (2009) Extraction and analysis of colourful
eggshell pigments using HPLC and HPLC/electrospray ionization tandem mass
spectrometry. Biomed Chromatogr 23: 602–606. doi:10.1002/bmc.1158.

5. Belshaw R, Pereira V, Katzourakis A, Talbot G, Paces J, Burt A, Tristem M (2004). “Long-term reinfection of the human genome by endogenous retroviruses”. Proc Natl Acad Sci USA 101 (14): 4894–99.

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2 Responses to The Science Behind Blue Eggs

  1. Jesse says:

    Has anyone looked into whether the biliverdin has some function in the eggshell, like antimicrobial or something? Guess if it does, it must not be that significant or else blue eggs would be more common.

  2. So far, it’s unclear what the purpose of biliverdin is in the eggshell. There was one study showing that blue-green eggs are associated with species that have two parents taking care of the offspring. It was also associated with species that don’t nest in a cavity in the ground. But why would those things be the case? No idea. There is some speculation that biliverdin could be used as an anti-oxidant, but it’s not clear whether that would be an advantage on an eggshell.

    The brown egg pigment, on the other hand, has antimicrobial properties. It might also affect how air passes through the shell when the chick is developing.

    Cassey P et al. 2012. Why are birds’ eggs colourful? Eggshell pigments co-vary with life-history and nesting ecology among British breeding non-passerine birds. 106:657-672.

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