Candling Eggs (With Pictures)

It is necessary to candle eggs for fertility when you are incubating eggs artificially using an incubator. Infertile or bad eggs can be discarded so that there is no risk of them going bad and exploding inside the incubator, contaminating the other eggs and if you are using a separate incubator or hatcher for the last few days of incubation to hatch your eggs, the extra space can be used for more eggs. Candling does not damage the embryos inside the eggs as long as you don't heat the egg up too much with the heat from the candling device.

Candling gets its name from days gone by when people used to use candles as the light source of course these days, you can buy or make your own candler using a light bulb as the light source. The cheapest way to create a DIY candler is to place a light bulb (low energy light bulbs are best as you can get a very bright bulb that doesn't generate as much heat as a conventional bulb) 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 onto this hole and turn the light on. 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 much like a spider's legs, bad eggs (sometimes called dead germs) where the embryo starts to develop but later died and infertile eggs that are clear except for the shadow of the yolk.

Dark shelled eggs are much harder to see through so you will need the brightest light source you can get and if you still can't tell, you will have to wait longer until the embryo has developed further and can be seen more easily.
Eggs are normally tested after 5 to 7 days of incubation. The most critical period of incubation during the development of the embryo is the first week so it is best to be patient and only take a look after the first week. You will see more after a week and can be more certain about the fertility.

Here are some photos of candling eggs which will help you to know what to look for.

Candling an Infertile Egg. The Yolk can't be seen as it is the opposite side but can sometimes be seen as a dark cloud towards the center of the egg.	Candling a Fertile Egg. Day 8. The Embryo is the dark patch in the center, blood vessels can also be seen. Notice the air sack at the top
Candling a Fertile Egg. Day 8. This is a dark brown Marans Egg. It is more difficult to candle and needs to be done in the dark to be able to see anything.	Candling a Fertile Egg. Day 8 The Embyo is towards the center / back of the egg and is not a prominent but blood vessels can still be seen.