How to Candle Eggs

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How do you know what’s going on inside your incubating eggs? You “candle” your eggs by shining a bright light against the shell and getting a glimpse of the interior goings on.  

What Is A “Candler?”

Called “candelers” because candles were originally used, the contemporary versions are more powerful, but won’t hurt your eggs. You can purchase candlers for a few dollars, useful for small flock owners, to ones worth one hundred dollars for large flock producers. What makes candlers unique is that they shine a very bright light without heating up the egg and risking damage.  

How To Use a Candler 

Candlers can be hand-held or sit on a flat surface. Place the large end of the egg, where the air cell is, against the candler. You’ll see the air sac at the bottom as a bright space. Above that, if the egg is fertilized, you’ll see a network of veins coming out of a dark blob close to the center of the egg. If the egg isn’t fertilized, there won’t be veins or the blob. By seven days, if eggs haven’t developed an embryo, they should be removed from the incubator. You can continue to candle eggs, judiciously, to check on development. But remember that there are periods, discussed below when you should not be opening the incubator.  

Unfertilized eggs can rot, giving off gases that can affect the other eggs. Rotten eggs can also explode, which is a mess you don’t want to clean up.  

Embryo Development 

Once you put the eggs in the incubator and start the process, eggs develop rapidly. During the first 24 hours, the embryo weight for a heavy-breed-breed chicken is .0002 grams, and the eyes are beginning to develop. Heart tissue begins to develop at 25 hours, and around 42 hours, the tissue starts emitting electrical impulses. 

 
 
Below is a list of the developments that you can expect to see in normal embryo development.  

Day 3 The structure for the beak, and the leg and wing buds appear. The embryo turns 90 degrees, with the yolk on its left side.  

Day 4 Eyes are becoming visible, which might show up as a red spot with candling.  

Day 5 Sex becomes genetically distinguished – hen or rooster. 

  Day 7 Knee and elbow joints have developed and digits are beginning to appear. The heart is now enclosed in a tiny thoracic cavity. 

Day 9-10  The structures around the eye continue to develop, such as the eyelids.  By Day 10, feathers are developing and the beak has grown and hardened. All these developments are still tiny. If you candle around this time, you’ll see well-developed blood vessels.  

Days 13-14 Weighing in at 7.39 grams, the original tiny blob has doubled its weight over 15 times! The claws are developing and the embryo is moving again — toward hatching position. Candling will reveal that the egg space is more than half filled by the embryo, and light doesn’t penetrate the dark area.  

Days 18-19 The yolk sac is gradually being drawn into the embryo’s body, providing nutrients that the chick will need upon hatching.  

Days 20-21 If you candle at this stage, you’ll see that there is a membrane all the way around the embryo. By Day 21, you may also notice some internal “pipping”, where the embryo is using its beak to poke through the membrane and is now breathing the air in the air sac. 

After 21 days, the embryo pushes its beak through the air sac while the allantois dries up as the embryo starts to use its lung to breathe. Using the “egg tooth” or sharp horny structure on the upper beak, the embryo pips or pecks at the shell. Once the shell has been pierced, the chick will now be breathing outside air, and will start to “zip” or break open the shell enough to flop out. The whole process of pipping and zipping can take 12 to 18 hours. Try not to handle the egg during this process as the embryo has carefully positioned itself to be able to escape.   

 
When To Candle 

 Limit your candling.  While it’s terribly tempting to take lots of peeks, the less you handle the eggs, the better. It’s generally recommended not to candle a single more than two or three times: once before placing an egg into the incubator, at seven days to check for development, and at 18 days to make sure that only viable eggs go into a hatcher, or when you shut the incubator down to maintain humidity. That first check lets you look for any micro-cracks in the shell that could lead to contamination of the embryo. 

If you’ve never candled before, you can practice on unfertilized eggs, to get used to handling the delicate ovoid and not pressing the light too hard against it. Oh, and you’ll want to watch a bunch of YouTube videos to learn different techniques.  
 
Dispose of any eggs that look cloudy or have a brownish tinge. After you have set the eggs to be incubated, if you have any doubts about how an egg is looking the first time you candle it, leave it for a couple of days and try again. You’ll rapidly train your eye to see the veins and embryo earlier to know which eggs are viable.  

Carla Tilghman is an urban chicken lady, raising primarily egg layers. Whether ordering fertilized eggs or working with a farm friend to breed heritage hens, she enjoys the whole incubating experience. In addition to weaving and knitting, she’s the editor of Backyard Poultry magazine.  

Originally published in the October/November 2022 issue of Backyard Poultry and regularly vetted for accuracy.