If you have looked into chickens at all, you will have noticed that chickens can lay eggs in a variety of colors. Check out the graphic below of all the different colors of eggs and the breeds from which they come.
We all normally immediately think of white eggs, right? That’s because the breed of chicken that lays white eggs – White Leghorns – are very prolific which makes them ideal for the large scale production needed for the U.S. food system. So let’s start with those white eggs. The white eggs are white because the shell is made of calcium, like bones. Bones are white, the shells are white. That’s an easy one, right?
But what about all of those other colors? The rest of the colors are the results of the hen’s genetics (her breed). As the egg works its way through the hen’s oviduct (if you didn’t see my blog on that, look here) pigment is added to the shell. If you’ll remember, the shell is added in the shell gland. The pigmentation in the form of porphyrins are synthesized in the shell gland and added to the shell as it is added to the egg. Depending on which part of the shell gland the porphyrins are added in, will determine if the egg shell color is the same on the inside as it is on the outside. Look at this:
This is super cool! The earlier (more anterior, closer to the isthmus) the porphyrins are added in the shell gland the deeper the color penetrates the shell. That’s why the Ameraucanas have blue on the inside and the outside of their shell! In the Ameraucana hen this egg came from, the porphyrins did not get equally distributed leaving the egg with spots of white on the inside. The pigment is deposited throughout the shell gland as seen by the Ameraucana, however 50 to 74% of the pigment is added in the last 5 hours before lay! Remember that the egg is in the shell gland for 20 hours.
Now that we understand how and why eggs can be different colors, let’s dig a little deeper into this word I keep using: porphyrins.
Porphyrins are a class of pigments. In addition to chicken eggs, porphyrins are also responsible for the colors of chlorophyll (green in plants) and heme (red in blood). *Side Bar: In blood, the nitrogen in the center of the porphyrin ring (containing 4 pyrrole subunits) allows an iron molecule to be added, resulting in the red color of blood. In chlorophyll, the center atom is Magnesium and it with the difference in side chains allows a difference in the absorption spectrum of light, which is why we see so many different greens in plants. I know that was A LOT of chemistry, but I am really excited to have learned that! Look at the pictures below. Notice anything similar between the structures shown? That’s the porphyrin ring!
There has been TONS of research done to determine where these porphyrins come from, can they be inhibited, and what they look like. I found this super review article, find it here. Egg color is something that is studied and selected in industry because an attractive product will sell. Therefore, things that may affect that color are of high interest to producers. Nutrition plays a huge role in the color of the eggs from white calcium deposits on the outside of the egg to variations in egg color within the same group (and same breed) of hens. While free range hens are touted to be producers of better eggs, the egg color is harder to control because of the variation of their diets. How cool is that!! Genetics, age, stress, and medications can also affect egg color. Egg color lightens with age. Additionally, at the beginning of each laying period (remember birds are seasonal layers, so when they resume laying in the spring and cease in the late fall/early winter) eggs are lighter in color and get darker as the laying period progresses. Stress from handling, housing and social dynamics, and the environment (e.g. heat) all affect both laying frequency and egg color.
If all of these things can impact egg color, imagine how they can affect other physiology of the chicken!
I’ll leave you with a poem as parting words:
Ode to Red, White, and Blue Eggs
There are many breeds of multicolored hens; They lay eggs of every hue. Their uteri filled with pyrrolic pens Paint the shells red, white, and blue!
So ask not "what came first, the chick'n or egg?" Nor "what can either one do for you?" Seek life's mystery of porph'rin and bile pigment, It is bound up in the red, white, and blue!
Found in: Schwartz S, Stephenson BD, Sarkar DH, Bracho MR. 1975. Red, white, and blue eggs as models of porphyrin and heme metabolism. Ann. NY Acad. Sci. 244:570–88. https://doi.org/10.1111/j.1749-6632.1975.tb41555.x
Review Article citation:
Wilson, PB. 2017. Recent advances in avian egg science: A review. Poult. Sci. 96(10):3747-3754, https://doi.org/10.3382/ps/pex187.