Candling is the process of using a strong light to illuminate the interior of the egg so you can see the development inside. It is one of those things we do, not necessarily because we have to, but because it brings the miracle of life right up close. It's fun and informative.
Candling your eggs is also a good way to find those that definitely will not hatch early on so they can be removed from the incubator. It's much less traumatic now, too. Leaving a bad egg in the incubator could cause lots of problems if it explodes. Yes, it could actually explode leaving a real mess in the incubator not to mention preventing the rest of the batch from hatching because of bacterial infection.
You can purchase an egg candler or you can make one very easily. Either way, the light must be bright and cool.
  A few things to keep in mind when you candle your eggs;
  • The heat produced by some bulbs is hot – too much candling can kill the embryo.

  • Other than candling at setting time, day 8 through 10, day 14, and day 18 are the days that would be most helpful.

  • At day 18 you should definitely see development. You can decide on this day whether to continue incubating the egg in question.

  • Do not candle on day 19 or after.


 Candle Drawing   To be able to see clearly inside the egg, you should look at the egg at approximately a 45 degree angle to the light source. You should be able to see the yolk – embryo (at early stages) and air sac.


Do this in a darkened room or at least with a dark background so that the sunlight or artificial light is not shining directly on the egg.


As with anything new, the more you experience this the better you will be able to tell what is really happening. Candling does not hurt the embryo so this can be done numerous times. Do not do every day though as this will only keep the eggs at a cooler temperature which will ultimately cause hatching problems because they are being taken out of the incubator too often.



Below are some examples of what you should look for:

 Good Egg  A faintly visible yolk in the centre with an air space : This is a freshly laid unfertilized egg. It is approximately a week old or younger, but it has not been in contact with a rooster.

 Non Fertile Egg   This egg is only a few days oldand undefined however will probably be good.  The yolk shows up fairly clearly, and there are no abnormalities.


 Fertile Egg   Thin red veins in a yellow mixture: This is a fertilized and developing egg at about 4 days. This is what you will see for a fertilized egg up until about 10 days. The veins will increase to a point where the chick's development will actually fill up the space available and cause the shell to become opaque.


On around day 12 to 16   you can actually see the chick move!

Opaque, dark mass in shell, unable to see through: This is a fertilized egg. The chicken growing inside has blocked light from passing through. This egg should hatch after the prescribed amount of time for the species you are trying to raise. If it does not, the chick has probably died inside the egg and the egg should be disposed of.

Opaque, but with a large air bubble off to one side of the egg: This is an egg that was fertilized, but the chick has since died. It can be disposed of.

Not opaque, but no yolk, and a large air pocket: This is an old, unfertilized egg. It can be disposed of.


Several things to look for which indicate bad eggs;


Bad Egg  Notice the dark ring – This egg is no longer viable as the ring is an indication of concentrated bacteria which has invaded the eggs' membrane. Sometimes all you see is a dark spot which appears in the middle of the yolk area. Wait a few days and candle again to see if spot has grown and ring begins to appear. If yes, then discard the egg.

Air Sac Drawing  The drawing at left shows the development of the air sac as the developing egg loses moisture. The air sac should be fairly obvious.

This is also a good way to be aware of humidity in your incubator. If not using a wet bulb thermometer or hygrometer, the air sac gives an indication of humidity in the incubator. Too little humidity and sac will be larger than it is supposed to be – too much and it will be smaller.

It is much easier to keep tabs on humidity with a wet bulb thermometer than to reply on the size of the air sac to gauge humidity readings. The air sac size is an approximation – the wet bulb will give you a reading. As your experience grows this will become more evident to you.


When candling on days 14 through 16 you will be able to pretty easily be able to notice the development of each egg. If this is your first time doing this I would suggest a couple things that might help.


1. Only use a pencil to mark eggs. I use coloured pencils to mark eggs with potential problems. A marker or pen will seep into the egg shell and very possibly kill the embryo.

2. Keep a journal of your progress. Mark all eggs that are possible problems and write this in your journal. Next time you candle you will remember what the marks mean and which eggs are which!

3. Handle the eggs carefully sp as not to scramble the contents.

4. Only keep them out of the incubator long enough to get the job done. You don’t want to have everything go sour just when it begins to get exciting.

5. Always discard the bad eggs. A rotten egg in the incubator can be a very smelly mess and potentially a hazard to all the viable eggs in the incubator.

6. Above all – Have Fun!