The sight of green chicken eggs often piques curiosity, as most people are accustomed to common white and brown varieties. This unique coloration is a natural phenomenon, stemming from biological processes within the hen. Green eggs highlight the genetic diversity found in poultry.
The Science of Eggshell Color
The green hue of certain chicken eggshells results from specific pigment deposition during egg formation. Biliverdin, derived from hemoglobin breakdown, is the primary pigment for blue or blue-green coloration. This pigment is synthesized in the hen’s shell gland and deposited onto the eggshell as it forms.
Eggshells are not simply painted on the surface; biliverdin permeates all shell layers. For hens that lay green eggs, biliverdin combines with protoporphyrin IX, a brown pigment also synthesized in the shell gland, to create the green appearance. The ratio and concentration of these two pigments determine the exact shade of green, from light olive to darker, mossy green.
Genetic Basis for Green Eggs
A hen’s ability to lay green eggs is governed by a dominant genetic trait. This characteristic links to a specific genetic alteration: the insertion of an endogenous avian retrovirus (EAV-HP) near the SLCO1B3 gene. This retroviral insertion influences biliverdin production and deposition in the hen’s shell gland.
Hens with this dominant gene produce eggs with blue pigment throughout the shell. This blue pigment, combined with brown protoporphyrin IX, results in a green-shelled egg. Chickens laying white or brown eggs lack this genetic insertion or express relevant genes differently, leading to absent or lower biliverdin concentration.
Chicken Breeds That Lay Green Eggs
Several chicken breeds are known for laying green eggs. The Araucana, from Chile, is well-known for its blue or blue-green eggs. Its derivative, the Ameraucana, also lays eggs in shades of blue or blue-green.
Hybrid chickens include Olive Eggers and Easter Eggers. Olive Eggers are a cross between a blue egg-laying breed (like an Ameraucana or Araucana) and a dark brown egg-laying breed, resulting in various shades of olive green eggs. Easter Eggers are mixed-breed chickens carrying the blue egg gene, allowing them to lay eggs in a wide spectrum of colors, including blue, green, pink, or brown.
Common Misconceptions About Green Eggs
Eggshell color—white, brown, or green—does not indicate differences in nutritional content, taste, or safety. Green eggs are as wholesome and safe as any other egg, containing the same vitamins, minerals, and protein. Shell color is purely a genetic trait of the hen.
No scientific basis suggests green eggs are superior in quality, taste different, or indicate a specific diet or freshness. The egg’s internal contents, including yolk and albumen, are unaffected by shell pigmentation. Any variation in taste or nutritional profile among eggs stems from the hen’s diet and overall health, not shell color.