Candling Chicken Eggs

I never seem to tire of incubating and hatching chicks, it’s always an amazing moment seeing chicks emerge from their shell and start peeping. My current batch of Light Sussex chicken eggs are 8 day into incubation and this week I have been candling them. Here’s my tips for candling chicken eggs.

Candling eggs is a very exciting time. Candling a week after incubation allows you to find out if an egg is fertile and allows you to remove clear eggs. Candling again after another week allows you to remove any ‘dead in shell’ embryos, those that haven’t developed further and usually only show a blood ring inside.

Candling Egg
Candling an egg using only a torch and my hand. The Embryo can be seen clearly inside. I could see a little more detail than can be seen in the photo which is slightly over-exposed.

A dark room and a bright torch is all you really need, even an old toilet-roll cut to fit the shape of the egg and held up towards the sun will work well.

With a freshly laid egg, you will see an almost clear egg. There is a feint shadow which is the egg yolk. This is called a ‘Clear Egg’ and is how the egg will remain during the first week of incubation if it is not fertile.

As the embryo develops, over the next few days, a very small darker patch will develop on the ‘shadow’ of the yolk. This is the germainal disc and takes an expert eye to spot this.

As time goes on, blood vessels begin to develop and become thicker and more visible and the embryo grows in size, until day 8 when the eye and the heart have developed and the embryo is starting to take the shape of a chick. If you are lucky, you may see the little peeps heart beating!

What’s going on in there?

So if like me, you’re curious as to what is happening inside the egg when you candle it, you’ll probably like the little overlay I added to one of my photos I took whilst candling my eggs. I turned the photo into black and white so the colour overlay is clear.

Candled Egg
So what is it we are seeing inside the egg when we candle it? I sketched this over the top of the image to show the approximate development inside the egg at day 8 when I am candling.

The air sack is easy to see. This grows as the egg develops and contains the first few breaths of air the chick will take when it pokes it little beak through on day 21.

In blue, I have highlighted the sack that is attached to the embryo called the allantois. This is situated around the back and head of the embryo and holds the waste products. When candling, it normally blends in to the dark shadow you see in the center of the egg.

The yolk sack itself I have shown in red (why? Because yellow didn’t show up too well and the yolk sack is so full of red blood vessels, it’s now pretty much all red).

The blood vessels emerge like spiders legs into the alubumen (egg white), the thicker of these can be seen when candling your eggs.

I have uploaded this photo to our page on candling eggs on the poultrykeeper website where there are a number of photos of good and bad eggs that I have candled.

So that’s it! I hope this helps you to understand what you’re seeing when candling eggs around day 8.

Please let me know what you think and tell me about your experiences.

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Tim Daniels

Tim is the founder of the poultrykeeper website and lives in Herefordshire, UK. He keeps Cream Legbar chickens, Silver Sebright bantams and hybrid layers for eggs, Abacot Ranger ducks, Brecon Buff geese and some quail.

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