The sight of a green egg often sparks curiosity, as most eggs are white or brown. While these traditional colors are common, certain animals, particularly birds, naturally produce eggs with a distinct green tint. This unique coloration is a biological outcome, determined by genetics and the intricate process of egg formation. Understanding why some eggs are green reveals insights into avian biology and diverse strategies.
Chickens: The Most Common Green Egg Layers
Chickens are the most widely known domestic animals to lay green eggs, with several breeds recognized for this trait. The Araucana, originating from Chile, is a prominent example, prized for its blue or greenish-blue eggs. This breed’s distinctive egg color stems from a dominant gene, believed to have integrated from a retrovirus. Araucanas are also characterized by unique feather tufts around their ears and can be rumpless, meaning they lack a tail. They are consistent layers, producing 150 to 250 eggs per year.
Another popular green egg-laying chicken is the Ameraucana, developed in the United States from Araucana lineage to refine the breed. Ameraucanas primarily lay blue eggs, though some produce a pastel green shade, and they are recognized for their muffs and beards. “Easter Egger” chickens are not a specific breed but are hybrids, often a cross between a blue egg-laying chicken (like an Ameraucana or Araucana) and a brown egg-laying chicken. This crossbreeding results in a variety of egg colors, including various shades of green, blue, pink, or even brown. Olive Eggers, a type of Easter Egger, produce darker olive-green eggs from a cross between blue and dark brown egg layers.
Why Are Some Eggs Green?
The green coloration of eggshells is primarily due to the deposition of a pigment called biliverdin. All eggs initially begin as white, with the shell composed of calcium carbonate. As the egg travels through the hen’s oviduct, specifically in the shell gland, pigments are synthesized and deposited onto the shell. Biliverdin, a blue-green pigment derived from bile formation, is integrated into the eggshell, giving it its characteristic color.
The presence and concentration of biliverdin in the eggshell are genetically determined. For instance, the dominant blue egg gene in Araucana chickens causes biliverdin to be deposited throughout the entire shell, so the color extends through to the inside. When a chicken with this blue egg gene is crossbred with a chicken that lays brown eggs, colored by a different pigment, protoporphyrin, the combination of blue and brown pigments results in various shades of green. This process occurs in the final hours before the egg is laid, as the shell is calcified and finalized.
Other Animals That Lay Green Eggs
Beyond chickens, many other bird species lay eggs with green or bluish-green hues. Many wild bird species exhibit this trait, often as camouflage to blend with their nesting environment, such as foliage or rocky terrain. Examples include the American Robin, known for its robin’s egg blue, which can sometimes appear greenish-blue, and the European Starling. Mallard ducks also lay eggs that can range from white to a pale pastel green.
Larger, flightless birds like the Southern Cassowary and the Emu, both native to Australasia, lay striking dark green eggs. These deep green eggs help them blend into the dense undergrowth of their habitats. Some heron species, such as the Green Heron and Purple Heron, produce pale blue-green eggs that are camouflaged within their nests built in trees or marshy areas. Certain amphibians, like the green salamander, lay clutches of green eggs, and spotted salamander eggs can also develop a green glow due to a symbiotic relationship with algae that aids embryo development.