The colour of eggshells

The color of an eggshell is primarily determined by the genetics of the bird species that laid the egg. Different bird species have specific genes that control the pigmentation of their eggshells, resulting in a variety of colors and patterns.

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1. Pigments: The two main classes of pigments responsible for eggshell color are biliverdin (blue-green) and protoporphyrin (brown). The presence, absence, and combination of these pigments determine the color of the eggshell.

• White Eggs: Eggs with white shells have no pigment on the shell. This is due to the absence of biliverdin and protoporphyrin.
• Brown Eggs: Brown eggs are the result of the deposition of protoporphyrin pigment on the eggshell.
• Blue or Green Eggs: Blue and green eggshells are the result of biliverdin pigment, which is typically overlaid with a white or brown pigment. The intensity of the blue or green color can vary.

2. Genetics: The genetics of the bird species dictate which pigments are present in the shell gland (uterus) during eggshell formation. Different breeds and individual birds may have variations in their genetic makeup, leading to differences in eggshell color within the same species.
3. Diet: While genetics play the primary role in eggshell color, the diet of the bird can also influence the intensity of the color. For example, a diet rich in certain pigments, such as xanthophylls and carotenoids, can enhance the coloration of brown or yellow egg yolks, which may indirectly affect the perception of the eggshell color.
4. Age: In some bird species, the color of the eggshell can change with the age of the bird. Younger birds may produce paler eggshells, which can become darker as the bird matures.

It’s important to note that eggshell color has no impact on the taste or nutritional value of the egg. Different eggshell colors are simply a result of the genetics and pigments present in the bird species.

References

• Hargitai R, Boross N, Hámori S, Neuberger E, Nyiri Z. Eggshell Biliverdin and Protoporphyrin Pigments in a Songbird: Are They Derived from Erythrocytes, Blood Plasma, or the Shell Gland? Physiol Biochem Zool. 2017 Nov/Dec;90(6):613-626. doi: 10.1086/694297. PMID: 28937855.
• Samiullah S, Roberts JR. The location of protoporphyrin in the eggshell of brown-shelled eggs. Poult Sci. 2013 Oct;92(10):2783-8. doi: 10.3382/ps.2013-03051. PMID: 24046428.