Written by Raymond Nakamura
Once upon a time (before the Internet), Raymond earned his doctorate studying the hydrodynamics of sand dollars. Nowadays he rents his brain at raymondsbrain.com for writing, cartooning, and thinking, when he is not washing the dishes, walking the dog, or helping his daughter with homework. Follow him on Twitter @raymondsbrain.


Egg colour is genetically determined

Created date

Monday, November 12, 2018 - 12:00am

Ever Wonder About Egg Colour?

I was buying some eggs the other day and noticed how abundant brown eggs now seem to be. That might just be me, or the places I shop. I wondered why eggs might come in white and brown.

Egg Stat See

In Canada, we consume about 8 billion eggs a year. At first that sounded like a crazy amount. Then I remembered that my own family of three goes through a dozen eggs a week. Expanding that math, (and assuming there are 37 million people in Canada), you get 7.7 billion eggs. Pretty close!

Tight fitting genes

Contrary to what some people believe, the hen’s living conditions do NOT affect the colour of the egg it lays. The colour depends on the genetics of the hen, which tends to be consistent with her overall coloration. Red, brown, or dark-feathered hens with red ear lobes generally lay brown eggs, while white or light-coloured hens with white ear lobes are more likely to lay white eggs. As they age, brown egg-laying hens tend to produce larger and lighter coloured eggs.

Free Ranging Market

Brown eggs tend to cost more than white eggs. There are two main reasons for this. First, the breeds that lay brown eggs tend to be on the larger side. They need to eat more than the average hen, so they cost more to raise. Second, egg producers using free range or organic methods tend to choose brown egg laying breeds, possibly because consumers seem to think that brown eggs are healthier. 

Quality Control

The type of feed, exposure to sunshine, and freshness can have an effect on the flavour and quality of an egg. (That would be whole other post!) So you might prefer brown eggs or white eggs, but colour is not an inherently reliable indicator of quality. So it is sort of a chicken and egg story, even though technically, the egg came first because the dinosaurs laid them.

When Pigments Fly

Speaking of which, at least some dinosaur eggs were probably blue. A few rare breeds of chicken also lay blue and green eggs. Brown eggs are made with a pigment called protoporphyrin, which is connected to heme molecules that give blood its red colour. Blue and green eggs have a pigment called biliverdin, which is another variation on the heme molecule that gives you the blue-green colour of bruises. These two pigments account for the range of egg colours found in the hundreds of chicken breeds.
Egg colours in the rest of bird-dom includes a broader range of hues, but they are also attributable to combinations of these two pigments and the basic white of the calcium carbonated shell.

Yolk Tales

Does it matter to the chickens whether their eggs are white or brown or green? According to genetic studies, the ancestors of domestic chickens include the Red Jungle Fowl (Gallus gallus), which lays white eggs, the Grey Jungle Fowl (Gallus sonneratii), which lays cream eggs (not to be confused with the ones that come out at Easter), and Ceylon Jungle Fowl (Gallus lafayette) that lays brown speckled eggs. Something sneaky must have happened with the breeds that lay the green and blue eggs.

Eggs-istential Threats

We began domesticating chickens around ten thousand years ago in Southeast Asia and apparently cockfighting was a big part of it. So at first, chickens were selectively bred for the characteristics of the male, whereas now, it is mostly about the females.

Beyond the intense competition of the supermarket, egg colour can have ecological significance in various ways. Birds that lay eggs in cavities or sit on them most of the time, tend to have white eggs, possibly so they are easier to see in the dark, or because white is the default colour of the calcium carbonate shell. Eggs on exposed nests tend to have brown speckled eggs, possibly so they are better concealed. In some cases, such as in ostriches, the eggs are in an exposed nest but are white. This may keep them overheating in the sun, while the parents are mostly able to defend against many potential predators.

Nowadays, chickens have farmers to both guard and steal their eggs, and the egg colours we see is a kind of shell game that depends more on market forces than natural selection. Do you buy eggs? If so, how do you decide which ones to get?

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