Eggs Feeding Genetics Stories From The Flock

What Makes Coloured Egg Shells?

Did you know there are only two genes for egg shell colour?

Yup, all those colourful eggs really started off with white shells. All egg shells are made of calcium carbonate which creates white shells. Coloured shells are affected by chemicals which add pigments to those white shells.

Brown eggs start off white with one, or more, brown overlays caused by protoporphyrin IX. It’s applied in the last 4-6 hrs before they are laid. Next time you crack open a brown egg check out the inside of the shell – it’ll be white.

Blue eggs are created by the pigment oocyanin, which permeates the shell so it’s blue on both inside and outside.

Green eggs are the result of the brown overlay protoporphyrin IX deposited over a blue oocyanin shell, so it will appear green on the outside, but blue on the inside. You might have to pick away the white membrane to see the blue shell colour.

Chickens carry two genes for egg shell colour (one from each parent), either white or blue, and potentially up to 13 genes that influence the brown overlays – that’s why there are so many different shades of browns and greens.

egg gene colour chart

The egg shell colour has no bearing on its nutritional value. What’s important is what hens were fed. Pigments change the outside colour of an egg and another chemical changes the appearance of the yolk. Xanthophylls (related to carotenoids that make carrots orange) are chemicals that create the dark yellow-orange egg yolks. They’re found in marigolds, brussel sprouts, broccoli, zucchini, and dark leafy greens like spinach, kale and collards.

If you want blue or green egg shells then you’ll need to get yourself some hens with at least one blue egg gene (i.e. Ameraucana, Araucana, Cream Legbar, Easter Egger, Isbar), If you want dark yolks then make sure your girls have a varied diet rich in xanthophylls.

2 comments on “What Makes Coloured Egg Shells?

  1. Laurie Kay

    Fascinating! I didn’t know any of this, Claire. Thanks very much for the fascinating information. My chickens do ‘the brown thing.’

    Like

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