Avian egg colors, ranging from speckled brown to pure white, are a remarkable evolutionary display. This coloration serves multiple biological functions, primarily involving protection from predators and communication between parents. For birds that lay eggs in exposed or semi-exposed nests, the shell color is an adaptation that determines the survival of the developing embryo. The presence of green eggs points to a fascinating blend of specific biological chemistry and ancient genetic programming across diverse species.
The Source of Green Coloration
The vibrant color of a bird’s eggshell is determined by two primary pigments. Blue and green hues are created by biliverdin, a byproduct of heme breakdown. The second pigment, protoporphyrin, is responsible for the red, brown, and speckled markings.
An egg receives its color while traveling through the shell gland (uterus). If biliverdin is deposited, the egg has a blue base color. A pure green egg results from biliverdin deposited throughout the shell structure combined with an external layer of brown protoporphyrin pigment.
This layering effect is similar to mixing blue with a transparent layer of brown or yellow to create green. The final green color depends on the concentration of the blue base and the density of the brown overlay. Unlike brown eggs, where pigment only tints the outer surface, biliverdin permeates the entire shell thickness, ensuring a consistent blue or green color throughout.
Common Domestic Birds Laying Green Eggs
Green eggs in backyard flocks are often produced by hybrid chickens known as Olive Eggers. These birds are not a standardized pure breed but result from crossing a blue-egg layer with a dark brown-egg layer. The blue egg gene comes from breeds like the Ameraucana or Araucana.
The dark brown pigment comes from breeds such as the Marans or Welsummer, which deposit protoporphyrin on the shell’s exterior late in formation. When this outer brown pigment is laid over the inner blue shell, the visual mix creates a distinctive olive green. Breeders can manipulate the darkness of the olive color by using darker brown-egg-laying roosters.
Green-shelled eggs are also found in domestic waterfowl, particularly certain duck breeds. The Cayuga duck lays eggs that start a very dark green early in the season, fading to a lighter green later. Wild Mallards and domestic breeds like the Indian Runner can also produce eggs with a light blue-green tint.
Wild Birds Known for Green or Blue-Green Eggs
In the wild, green and blue-green egg coloration serves important ecological functions, often acting as camouflage or a protective barrier. The American Robin lays a famous, uniform blue-green egg, which is thought to conceal the clutch from predators in an open, sun-dappled nest. For species with exposed nests, the biliverdin pigment may also help shield the developing embryo from ultraviolet radiation.
Ground-nesting species in tropical regions also exhibit striking green eggs adapted to their surroundings. The Cassowary, a large flightless bird native to New Guinea and Australia, lays huge, deep emerald-green eggs. This rich green blends seamlessly with the forest floor’s leaf litter, providing camouflage for nests that are shallow scrapes on the ground.
Another example is the Tinamou, a South American ground bird whose eggs are often glossy and vibrant, ranging from turquoise to emerald green. Unlike the Cassowary, the Tinamou’s conspicuous, shiny eggs may not be primarily for camouflage. One hypothesis suggests the bright color acts as a signal to females seeking a communal nest, or that the rapid color fading forces the incubating male to sit more diligently on the clutch.