Peruse¹ the egg section of a grocery store or farmers market, and you'll notice cartons of eggs separated into white and brown, sometimes even green or blue eggs. But once they arrive scrambled² on a plate, perhaps, or baked into a cake, it's tough to tell the difference. In fact, what is the difference between eggs with shells of different colors? 

 

去超市或农产品市场仔细观察成箱的鸡蛋，你可能会注意到鸡蛋既有白的，也有褐色的，有时候甚至是绿色或蓝色的。可是变成炒蛋或蛋糕之后，又基本看不出区别了。那么，这些鸡蛋除了蛋壳颜色不一样，还有什么不同呢？

 

This isn't a flour or rice situation. Brown eggs are not more "natural," and white eggs have not been decolored with bleach³. Both varieties occur completely naturally, as do bluish-green chicken eggs. But really, all chicken eggs are the same on the inside. So what causes different egg colors among the same type of bird? 

 

Dr. Justin Fowler, a professor in the University of Georgia's poultry⁴ science department, shines a light on things. "The different colors, or the presence of spots or speckling, come down to the genetics of the bird," he says. 

 

"Leghorn chickens (which make up most of the commercial egg industry in the US) lay white eggs, while Orphingtons or Plymouth Rocks will lay brown eggs," explains Fowler. "The Ameraucana breed has a pigment⁵ that is able to permeate⁶ the whole egg shell and make blue-colored eggs that are colored on both the inside and outside of the shell." 

 

Want to know whether a specific chicken will pop out a white egg or a colored egg? Examine the chicken's earlobe. (Surprise, birds have earlobes!) "Breeds with white earlobes will typically be those that do not put extra pigment on the egg shell before laying," says Fowler. And more often than not, chickens with lighter⁷ earlobes tend to have white feathers, and thus white eggs, while those with colored feathers and earlobes tend to produce colored eggs. 

 

Where does this color come from? Here's where understanding how an egg comes to be is important. Chicken yolks, or ova, form in the chicken's ovaries. A fully⁹ formed ovum will leave the ovary and be deposited into the oviduct. This part of the chicken's reproductive system has five distinct sequential segments which the yolk⁸ passes through on its way to the outside world, but it's the fourth one -- the shell gland¹⁰ -- that affects the color of the egg. This is where the shell forms around the ovum. 

 

"The shell of all chicken eggs are made of calcium¹¹ carbonate, a crystal that is white in color," says Fowler. "So all eggs are, at least at the start, white. Any that we then see that are other colors have had a pigment deposited on them as they were moving through the oviduct, after the white egg shell had been deposited. You can see this if you open up a brown egg and look at the inside of the shell, it'll be white." 

 

Two pigments¹² are responsible for the spectrum¹³ of chicken egg color. Shades of greens and blues¹⁴ are caused by the pigment biliverdin, while protoporphyrin is responsible for reddish-brown hues¹⁵. 

 

The same egg-coloring process holds true for all birds with colorful eggs. American robin¹⁷ (Turdus migratorius) eggs, for instance, are famously blue. The pear-shaped eggs of a common murre (Uria aalge) can sport a blue hue¹⁶, be speckled or feature brown streaks¹⁸.

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