Egg Candling


Just what is candling?  Is it difficult to do?  

Eggs are candled to determine the condition of the air cell, yolk, and white. Candling detects bloody whites, blood spots, or meat spots, and enables observation of germ development. Candling is done in a darkened room with the egg held before a light. The light penetrates the egg and makes it possible to observe the inside of the egg.

The candler should be set on a box or table at a convenient height (about 38 to 44 inches from the floor), so the light will not shine directly into the eyes of the operator. In candling, the egg is held in a slanting position with the large end against the hole in the candler. The egg is grasped by the small end and, while held between the thumb and tips of the first two fingers, is turned quickly to the right or left. This moves the contents of the egg and throws the yolk nearer the shell. Because of the color of their shells, brown eggs are more difficult to candle than white eggs.

To do a reasonable job, an extensive knowledge of candling is not necessary, particularly if the eggs are all relatively fresh. One should be able to distinguish a fresh egg from a stale egg and detect such abnormalities as bloody whites, blood spots, meat spots, and cracked shells. In a fresh egg, the air space is plainly visible and moves freely. The white is thin and clear. In a stale egg, the air space is plainly visible and moves freely. The white is thin.

Candling Incubated Eggs

Incubated eggs are candled to determine whether they are fertile and, if fertile, to check the growth and development of the embryo. White eggs should be tested for fertility on the third day. Brown shelled eggs on the fifth or sixth day because it is difficult to see the embryo clearly before this time.

A small reddish area with blood vessels extending away from it will be visible in fertile eggs. This is the embryo floating around inside the egg, looking like a huge red spider. If the embryo dies, the blood draws away from the embryo and forms what is called a blood ring. All clear eggs and eggs showing blood rings or streaks should be removed from the incubator. If eggs are not candled during the early stages of incubation, it will be difficult to determine whether the egg was fertile; embryos that die early soon decompose and are not easily distinguished from rotten eggs.

Candling is not extremely difficult as it merely consists of shining a light behind an egg in order to inspect the embryo and determine which eggs are fertile and which should be culled. The technique gets its name from literal candles as it was the only light source available.

The tricky part, at least for me, is actually determining without a shadow of a doubt that the egg in question is indeed a bad egg. Knowing exactly what to look for at the various stages or embryo development and trusting yourself to cull or remove the correct ones.

My incubator has a built in candling light that checks one egg at a time.  There are entire trays that can candle all eggs at once but the accuracy of this method is not as reliable.  Inspecting each egg one by one to check for abnormalities is a much reliable method.  

Finding a dead embryo, known as ‘clears’, is generally done within the first 5-6 days, however accuracy improves significantly when performed between 9 and 10 days instead. Any time before that and you actually risk making a bad judgment on an egg and marking a perfectly healthy embryo as a clear.

Performing this tactic is useful for a number of reasons, such as early detection of problems with the embryo, a reduction of hatchery waste, and an increase of overall quality in hatched chicks. By candling you can also discover hairline cracks, cracks in the shells that are essentially indecipherable by the naked eye. Oddly enough, studies have proven that the mortality rate of chicks from eggs with hairline cracks are significantly higher than those without. Therefore, a hairline crack should be grounds for the egg being marked as a clear.

With candling, you’re given a good idea which chicks will hatch without problems and which may not make it. Using this technique, it’s suggested that you rotate healthy eggs closer to each other and move the cleared eggs to the edge or altogether. The reason for this is because having eggs touching during hatching is important as the movement and vibration of the first chick to hatch will actually trigger the other chicks to begin hatching as well.

Candling is a tricky skill to pull off correctly, but it’s a necessity if you’re hoping to succeed in raising chickens year after year. Get familiar with the best methods and take notes. Sometimes the best way to learn is firsthand.


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