February 14, 2016

Candling Eggs

Candling is a method of checking an eggs fertility. Natural fertility is never 100%, in fact if you have bought shipped eggs for hatching the fertility rate is likely to be nearer to 50%, as even the best shipping method will have damaged some of the eggs. Otherwise if they are eggs from your own hens or ones that have never been shipped then you may expect up to an 80% success rate.

You will need to check your eggs for fertility when artificially incubating  using an incubator. Infertile or bad eggs should be discarded so that there is no risk of them going bad and exploding inside the incubator, which would contaminate the other eggs.


Making a Candler

Candling gets its name from the days when people used candles as their light source, of course these days you can buy or make your own candler using a light bulb as the light source. The easiest way to make your own candler is to place a light bulb (low energy light bulbs are best as you can get a very bright bulb, but without the heat of a conventional one) and light fixture inside a cardboard box. Cut a small, round hole in the top of the box, just big enough to sit the pointed end of your egg into. Place your egg into this hole and turn the light on.

Candling does not damage the embryos inside the eggs as long as you don’t heat them up too much with the heat from the candling device.



It is easiest to see development of the embryo after a week. The most critical period in the incubation process is the first week so if you do decide to candle your eggs before a week then be very careful with them and do not overheat them. Always clean your hands well before touching the eggs, otherwise you risk transferring harmful bacteria to the chick embryos. Limit your candling, to a very maximum of three times during incubation as each time humidity and heat is lost from the incubator, which could jeopardise your hatching success.

You should be able to see what is inside the egg and with practice you will be able to identify fertile eggs by the spider-like blood veins spreading out , bad eggs, (may have blood rings and will be cloudy) where the embryo started to develop but later died and infertile eggs that are clear except for the shadow of the yolk.

Always make sure your hands are clean otherwise you risk transferring harmful bacteria to the chick embryos, through the shells. Limit your candling, to a very maximum of three times during incubation as each time humidity and heat will be lost from the incubator, which could jeopardise your hatching success.

Don’t be too quick to discard an egg that does not appear to be developing. We recommend that you wait until Day 14 before getting rid of it. Development will be clearer by then and you will be more easily able to compare the egg in question with others of the same age.


Dark brown eggs

Dark coloured shells are more difficult to see through, so you will need a brighter light. Even with a brighter light you may need to leave the egg longer until the embryo is potentially bigger and hope that you don’t in the meantime have any eggs that go bad and explode.


Some Candling Photos

Candling is not a specific art, it is more of a comparison, meaning all the eggs of the same age should look the same. It is something best learned by doing it.


How do I spot bad eggs?

The egg on the left above shows a ring at 6 days. This ring is formed by concentrated bacteria which has invaded the eggs’ membrane. It can become present very early, or after the chick has already started to form, as in the picture on the right. In the picture on the right the ring, or portion of it, can be seen at the bottom of the egg with the expired chick in the middle.



For a different reason. The egg on the left above shows a blood spot. In my experience an egg with a blood spot will not hatch. They will go bad and explode. The egg on the right at 6 days shows ‘clear; which means it is infertile, or too old to germinate.


The egg on the left shows a blood spot incubated to 8 days. You can see the bacterial ring forming at this point. Soon this egg will start to ‘weep’ and if it isn’t caught in time, it will explode into a stinky mess. The egg on the right shows highly defined pores. Eggs that look like this under candling have a slim chance of hatching in my experience. I’ve noticed that it mostly depends on the severity of the porosity.



On the left, you can see the ‘spider’ of veins growing away from the embryo, this egg is 6 days old. The egg on the right is  2 weeks old, you can see the clear spot beneath, with the yolk and chic embryo floating at the top.