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The Science Of Eggs: Size, Shell And Yolk Colour

Blue eggs!?! Our Ameraucana chickens have started to lay! With the extra splash colour in egg flats, we’re getting questions from customers that we answer here.

Why are some eggs eggs smaller?

At around 20 weeks of age young hens, or pullets, start to lay eggs. Their first eggs – pullet eggs – are small but prized by chefs who consider them tastier. As a pullet lays more and eats the higher protein feed we give layers, her eggs get bigger (but remain sooo delicious!)

In addition to bird age, egg size varies with food quality and quantity, water availability and their overall body size and weight. For example, our hens lay smaller eggs when it is really hot and/or dry.

What gives the eggshells their colour?

White: Eggshell colour is genetically determined by a chicken’s code or “recipe”, like how our eye colour is determined by our genetic code. The default, or wild type, colour of an eggshell is white because it is made mostly of calcium carbonate (calcareous), along with other minerals and proteins.

Brown: Chicken breeds that lay brown eggs, like our ISA Browns, have a genetic code to produce an extra brown pigment that coats the eggs during the last few hours of egg development in the hen’s body. This pigment, called protoporphyrin, is derived from hemoglobin or blood. The typical brown egg like the ones we sell are produced from a medium amount of pigment. Lighter or darker brown eggs have less or more pigment.

Along with genetics, the intensity a brown egg is affected by stage in the laying cycle, with egg colour fading as the chicken approaches molting (her cycle for feather loss and regrowth). While the pigment is found mostly in the calcareous part of the shell, some is also in the outer layer. The pigment in the outer layer can washed and scratched from the shell, further changing the intensity of the brown colour (reference).  

Blue: The Ameraucana breed carries a dominant gene for blue eggshells;  in addition to calcium carbonate, their eggshells contain oocyanin that is a byproduct of bile and produces lavender to blue shell colour.  This blue pigment is not a coating like brown but is part of the shell, meaning the blue colour can’t wash away. Like brown pigmentation, the amount of oocyanin will lessen throughout a hen’s laying cycle leading to progressively lighter blue eggs. 

Olive: While we don’t have olive eggs on our farm, it’s worth describing their genetics too. Green or olive eggs comes from a blue eggshell covered by brown pigment! The darker the brown coating, the more olive the egg; varying degrees of brown on blue give a range called “Easter eggs”.  Easter eggers and Olive eggers are hybrids or “mutts” – usually a cross between an Ameraucana (blue eggshell) + a Maran (dark brown).

Finally, the genetics behind eggshell colour is not linked to feather colour. Our brown ISA Brown and white Chanteclers lay light brown eggs and our brown-ish multi-coloured Ameraucanas lay lavender eggs! 

What makes the egg yolks so orange?

Adding even more to the rainbow effect, egg yolks also vary in shades of yellow and orange. Here at 3R, we pride ourselves on eggs with deep orange yolks.

We care about yolk colour because it is not determined by eggshell colour; rather, it is determined by the hen’s diet: deep orange yolks produced by our pasture-raised hens come from green forage and other plant material with xanthophylls  pigments – a type of carotenoid (think orange!). During winter months we maintain the yolk colour (and nutrient-density) by feeding them sprouted grain.  

In contrast, yolk colour lightens when pasture growth slows during the heat of the summer. It’s amazing to see the hens’ physiology respond to their environment – and another great way for us to stay in tune with the pastures.

Get cracking and enjoy!!!


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