Cracking the mystery on chicken eggs

As we enter summer and more people are looking at raising backyard chickens, they may be curious why chicken eggs can be different colors. Most eggs are white or brown, but they also come in other colors such as cream, pink, blue and green. In addition — and this is no “yolk” — some eggs are speckled.

Nature has provided chickens with diverse color patterns for their feathers, skin patches and eggshells for various purposes, including camouflage to protect from predators and to denote individual identity.

The color of an egg is primarily determined by the chicken’s genetics, said Gregory Archer, Ph.D., Texas A&M AgriLife Extension Service poultry specialist in the Texas A&M Department of Poultry Science, Bryan-College Station. That means the breed of hen will usually indicate what egg color will be produced.

For example, Leghorn chickens lay white eggs, Orpington’s lay brown eggs and Ameraucana chickens lay blue eggs. And the “olive egger” breed lays … wait for it … olive-green eggs.

But appearances aside, all chicken eggs have no major differences in taste or nutritional composition, Archer said.

Chicken earlobes help predict egg color

A good way to guess what color eggs a chicken will lay is to have a gander at the hen’s ear lobes.  

“Generally, hens with white earlobes will produce white eggs,” Archer said. “But all eggs start out white because the shells are made from calcium carbonate. They get their color from the hen’s genetics as the egg forms.”

He said chickens with lighter earlobes often have white feathers and produce white eggs. Those with colored feathers and darker earlobes will likely produce colored eggs.  

Adding a little color

Nature has its own way of coloring eggs, and it doesn’t require food coloring or a paintbrush. Different eggshell colors come from pigments deposited onto the shell as the egg forms in the hen’s oviduct. The oviduct is a tube-like organ found along the hen’s backbone between the ovary and the tail.

A chicken yolk, or ovum, forms in the hen’s ovaries. A fully formed ovum leaves the ovary and makes its way into the oviduct. It then goes through a five-stage process to help ensure the yolk makes it safely to the outside world. The entire egg-forming process usually takes a little more than 24 hours.

It’s during the fourth stage of this process, which involves the shell gland, that pigments are deposited onto the shell, producing its color. So, in short, different breeds of chicken deposit different pigments on the shell as it forms, changing its exterior — and sometimes also its interior — shell color.  

Archer added that shell pigment also has anti-microbial properties that may help reduce the risk of embryonic mortality. 

 A pigment of the imagination

White Leghorn chickens lay white-shelled eggs and breeds like Plymouth Rocks and Rhode Island reds lay brown-shelled eggs. The shells are brown because a pigment known as protoporphyrin is deposited onto the shell. But because this happens late in the shell formation process, the pigment rarely penetrates the shell’s interior.

“This is why when you crack open a brown egg, you will see the interior of most of the shells remains white,” Archer said.

However, there is a notable exception. A pigment called oocyanin is deposited on the egg of the Ameraucana breed and can penetrate both the exterior and interior of the shell, making them both blue.

Other breeds, such as Araucana, Dongxiang and Lushi, lay blue or blue-green eggs.

An olive egger results from a cross between a hen and a rooster from a brown egg-laying and a blue egg-laying breed. The hen produces a brown pigment that penetrates the blue shell of the egg, resulting in a greenish-hued egg. The darker the brown pigment, the more olive-colored the egg appears.  

Other chickens that lay colored eggs include the Easter egger, Barred rock, Welsummer and Maran, with the color of the egg depending on the breed and its genetics.

A hen will only lay one color egg her entire life, so if she starts by laying blue eggs, her eggs will always be blue.

Some speculation about speckled eggs 

Speculation aside, the general consensus from the eggs-perts is the speckles on speckled eggs are just extra calcium deposits. One reason speckles are formed is the egg-shaping calcification process is disturbed. Another possible reason could be a defect in the shell gland.

Sound a little scrambled? Don’t worry about it … just keep your sunny side up and know there’s probably more than one explanation for this speck-tacular occurrence.

Oh, and although technically considered “abnormal,” speckled shells can sometimes be stronger than the average egg.

Egg-straneous factors influencing color, size and shape   

While genetics primarily determine egg color, other factors can also influence the color and other characteristics of the shell. These factors include a hen’s age, diet, environment and stress level.

“As they age, hens that lay brown-colored eggs may start to lay larger and lighter-colored eggs,” Archer said. “While this may produce an egg of a lighter or darker shade, it will not alter the egg’s basic color.”

While not directly associated with color, an oddly or irregularly shaped egg may occasionally pop out. This may result from a dysfunction in the hen’s egg-forming process.

Archer said both very old and very young hens are the most likely to lay abnormally shaped eggs.

“Stress factors like disease, heat or overcrowding may also affect the hen and impact the egg’s size, shape and quality,” he said. “A lot also depends on the amount of calcium the hen has in its body and can provide for the egg-making processes.”

All yolking aside: Color, nutrients and seeing double  

You may also be wondering if the color of the egg affects the color of the yolk. Well, it doesn’t, but the hen’s diet certainly can. For example, if a pasture-raised hen eats plants with yellowish-orange pigmentation, the yolks can take on a more orange color. If she eats mainly a corn- or grain-based diet, the yolk is more likely to be a pale yellow.

Research has shown darker, more colorful yolks have the same amount of protein and fat as lighter yolks. However, it has also demonstrated that eggs from pasture-raised hens can have more omega-3s and vitamins and less cholesterol than other eggs.

Speaking of yolks … this will crack you up. Sometimes, an egg will have two yolks. While some people think a double yolk is good luck, the reason is more mishap than fortune. A double yolk is a fluke that occurs when a hen ovulates too rapidly, releasing two yolks, usually about an hour apart. These yolks enter the oviduct and eventually wind up in the same shell.

Hormonal changes or a hyperactive ovary can also cause these double releases. These “double-yolkers” are most common among younger chickens due to their reproductive system not yet being fully developed.

Where can you learn more about chickens and eggs? Well, you could go to the “hen-cyclopedia” of course. But if you don’t have one handy, visit this publication on the AgriLife Learn website for more information.