Chickens can indeed lay green eggs, a captivating biological occurrence that adds a spectrum of color to the traditional egg basket. This distinctive eggshell hue is not a result of diet or environment, but rather a fascinating genetic trait found in specific chicken breeds.
How Chickens Lay Green Eggs
The process of eggshell coloration begins in the hen’s oviduct, specifically in the shell gland or uterus, where pigments are deposited onto the developing egg. All chicken eggs initially start as white, with the color being applied during the final stages of shell formation. Green eggshells result from a combination of two primary pigments: biliverdin and protoporphyrin IX.
Biliverdin, a blue-green pigment, is integrated throughout the eggshell during its formation, meaning the color permeates the entire shell, not just the surface. This pigment is a byproduct of bile production and is synthesized in the shell gland. The genetic basis for blue eggshells, which is a precursor to green, involves a dominant gene influenced by a retroviral insertion in the chicken’s DNA, affecting the expression of a membrane transporter protein (SLCO1B3) that carries biliverdin.
The green color then arises when a brown pigment, protoporphyrin IX, is deposited over this blue base. Protoporphyrin IX, which gives brown eggs their color, is applied during the last few hours before the egg is laid. The combination of the underlying blue from biliverdin and the superficial brown from protoporphyrin IX results in the various shades of green observed in eggshells.
Breeds That Lay Green Eggs
Several chicken breeds are known for their ability to lay green eggs. The Ameraucana chicken, a purebred recognized for its blue eggshells, carries the dominant blue egg gene (O) which is fundamental for green egg production when combined with brown egg genetics.
Olive Eggers are hybrid chickens specifically bred to produce olive-green eggs. They are typically created by crossing a blue egg-laying breed, such as an Ameraucana or Araucana, with a dark brown egg-laying breed like a Marans or Welsummer. The depth of the olive color can vary depending on the specific parent breeds used in the cross.
Easter Eggers are another popular type, though they are not a recognized breed but rather a mixed-breed chicken that can lay eggs in various shades, including blue and green. These chickens result from crosses involving birds that carry the blue egg gene, often from Ameraucana or Araucana ancestry. Favaucanas, a cross between Faverolles and Ameraucanas, also lay green to sage-green eggs.
Properties of Green Eggs
The color of an eggshell, whether green, blue, brown, or white, does not influence the egg’s internal properties, such as its nutritional content, taste, or safety for consumption. The pigments that color the shell do not penetrate the inner contents of the egg. A green-shelled egg is nutritionally equivalent to any other chicken egg, assuming the hens are fed a similar diet and live under comparable conditions.
The yolk and albumen (egg white) of a green egg will appear the same as those from white or brown eggs. The color of the yolk is primarily determined by the hen’s diet, with a diet rich in carotenoids leading to a darker, more orange yolk. Any greenish tint observed in a hard-boiled egg yolk or albumen is typically due to a chemical reaction during cooking, where sulfur in the egg white reacts with iron in the yolk, and is not related to the shell color. This greenish ring is harmless and does not indicate spoilage.