Naturally occurring green eggs are a striking anomaly in the animal world. Most avian and reptilian eggshells are shades of white, cream, or brown, making the emerald, olive, or soft sage hues particularly noticeable. This unique coloration represents a fascinating intersection of genetics, biochemistry, and evolution across different animal classes.
Common Poultry Breeds Laying Green Eggs
The most frequent source of green eggs comes from domesticated fowl, specifically certain chicken breeds and their crossbred descendants. The Araucana, a breed originating in Chile, is the foundational source of the blue egg gene in many green-laying birds. This gene causes the deposition of a blue pigment that penetrates the entire shell matrix. The green coloration is typically a result of crossbreeding a blue egg-laying chicken with one that lays brown eggs, creating a hybrid known as an “Easter Egger” or “Olive Egger.” The overlying brown pigment then mixes with this underlying blue to produce various shades of green.
The Pigment Responsible for the Green Color
The vibrant blue and green egg colors are directly attributable to a specific bile pigment called biliverdin. Biliverdin is a breakdown product of heme metabolism, deposited onto the eggshell as it forms within the hen’s oviduct. This deposition occurs early, allowing the pigment to permeate all layers of the shell, meaning the inside of a blue or green shell is the same color as the outside.
In contrast, the brown and reddish colors seen on many eggs are caused by a different pigment, protoporphyrin IX. The green hue of an egg is created by the precise combination of these two pigments: the blue biliverdin underneath and the brown protoporphyrin IX layered on top. The final shade of green depends on the quantity and concentration of the brown protoporphyrin pigment applied over the constant blue base.
Green Eggs Beyond Birds
While chickens are the most commonly known producers of green eggs, the coloration appears in other classes of the animal kingdom, often for different biological reasons. Certain reptiles, such as some lizard species, lay eggs that exhibit a light green or bluish-green tint. Unlike in birds, where the color is a metabolic byproduct deposited on the shell, the tint in lizard eggs may result from external absorption or the composition of the soft, leathery shell itself.
In the insect world, several species produce egg structures that are distinctly green. The Praying Mantis secretes a foamy, protective casing called an ootheca, which hardens into a pale green structure that houses dozens of eggs. Similarly, the eggs of the Green Lacewing are laid on tiny stalks and are initially yellowish-green, later turning bluish-green before they hatch. These non-avian examples demonstrate that green egg coloration is a convergent trait, achieved through varied biochemical processes.