Chicken eggs appear in a spectrum of colors, from common white and brown to unique shades of blue, green, and pink. The shells are fundamentally composed of calcium carbonate, which is naturally white. White is the baseline color before any pigments are applied during the shell-forming process. A hen’s genetic code determines the final color by dictating whether and how much pigment is deposited onto this white base.
How Shell Color is Formed
The process of forming an eggshell takes approximately 20 hours within the hen’s reproductive tract, specifically the shell gland. All eggs begin as white structures because the shell is almost entirely made of calcium carbonate crystals. Coloration is not introduced until the final hours, when pigments are secreted and applied to the shell surface.
Brown and red tones come from protoporphyrin, a pigment derived from the breakdown of hemoglobin in the hen’s blood. Since this pigment is deposited late in the process, it forms a surface coating, meaning a brown egg is white on the inside. Blue and green eggs are colored by biliverdin, a different pigment that is a byproduct of bile formation.
Biliverdin is applied earlier, allowing it to permeate the shell and color both the interior and exterior blue. Green eggs are produced when a hen that genetically lays blue eggs also applies the brown protoporphyrin coating over the blue shell. This layering results in a green or olive hue, with the intensity of the brown pigment determining the final shade.
The Genetic Link to Egg Color
The ultimate factor dictating an egg’s color is the hen’s genetics, which determine the type and amount of pigment produced. Breeds like the White Leghorn, the primary layer of white eggs, lack the genetic programming to deposit significant pigment, resulting in a pure white shell. Conversely, breeds such as the Rhode Island Red and the Orpington carry genes that instruct the deposition of protoporphyrin, yielding brown eggs.
A common indicator of a hen’s egg color is the color of her earlobes. Hens with white earlobes often produce white eggs, while those with red or brown earlobes tend to lay colored eggs. This is a correlation, not a direct causal link, as the traits are controlled by separate genes that are often inherited together.
Exceptions to the earlobe rule are numerous, particularly among heritage and ornamental breeds. For instance, the Ameraucana breed, known for its blue eggs, often has red earlobes. Genetic factors also influence unique colors, such as the blue-green eggs laid by Araucanas or the dark brown eggs produced by Marans chickens, which deposit a high concentration of protoporphyrin.
Egg Color and Nutritional Value
Shell color has no bearing on the nutritional content, flavor, or quality of the egg inside. This is a persistent consumer misconception, often leading to the belief that brown or colored eggs are healthier than white ones. The internal composition of the egg, including protein, fat, vitamins, and minerals, is identical regardless of the shell’s hue.
The actual determinants of an egg’s nutritional profile are the hen’s diet and living environment. Hens that are pasture-raised and consume a varied diet of grasses and insects produce eggs that can contain higher levels of certain nutrients, such as omega-3 fatty acids and specific vitamins. The color of the egg yolk itself, which can range from pale yellow to deep orange, is a direct reflection of the hen’s diet and the amount of carotenoids consumed, not the shell color.