What Determines the Colors of Eggshells?

The color and breed of the chicken determines the eggshell color. In other words, genetics controls the color. While the egg is formed in the reproductive tract of the hen, pigments are deposited on the shell. The brown pigment is ooporphyrin, and the bluish-green pigment is oocyanin. The brown pigment is deposited on the outside of the shell and can be washed or scratched off. The blue-green pigment permeates the shell and does not wash off. White hens usually lay eggs without pigments.

  1. Breeds

    • Rhode Island Reds and New Hampshires lay brown eggs. Rhode Island Reds and New Hampshires have reddish brown feathers. In general, brown or black colored hens lay brown eggs. White hens such as white Leghorns lay white eggs.

    Araucana Breed

    • Araucana chickens, a breed from South America, have a wide variety of feather colors but lay blue-green eggs. The gene that controls the blue-green color is dominant. If Araucana chickens are crossed with chickens of other breeds, the offspring usually produce blue-green eggs.

    Eggshell Structure

    • The uterus or shell gland adds the eggshell. The shell consists of 94 to 97 percent calcium carbonate and about 3 percent eggshell pigment and organic matter. Hens begin laying eggs at 18 to 22 weeks of age. It takes about 20 hours to form the shell on an egg.

    Individual Variation

    • Eggshell color varies from bird to bird. As a brown egg layer gets older, the color of the shell becomes lighter.

    Drugs and Egg Color

    • Nicarbazin is a drug that controls coccidiosis in chickens. Coccidiosis is a protozoan disease that causes bloody droppings. If nicarbazin is added to a brown egg layer's diet, the hen will begin to lay white-shelled eggs within 48 hours. The shells will once again be brown when the drug is removed from the diet. Low levels of nicarbazin cause a loss of color in the eggshells.

    Egg Content

    • If the chickens are fed a similar diet, brown and blue-green eggs have the same nutrients, shelf life and flavor as white eggs. Brown eggs are usually more expensive to buy because it costs slightly more to raise brown egg layers. The diet of the chicken does not influence eggshell color.