Candling eggs refers to the process by which one can observe the growth and development of the embryo present inside the egg. This can be done by observing the air space, egg white, and the yolk inside the egg. It also helps one to detect blood rings or meat spots in the egg. Candling at regular intervals can help one understand the growth and development of the egg, and separate the dead eggs from those that are fertile.


A candler is the source of light that is used to assess whether the egg is fertile or not. While candlers can be made at home, there are some excellent ones that are available in the market too. Small flashlights can also be used by regular breeders for this purpose. Candlers can also be made by putting a light source into a container and cutting a small hole into it, or by folding a thick sheet of paper into a cone around a light source with an opening at the tip from which light can shine out. Such homemade candlers are good for white or pale eggs. However, for spotted or dark eggs, high intensity candlers are commercially available.


This process should always be carried out in a dark room. The candler should be set at a convenient height so that it does not shine directly into the eyes of the observer. The egg should be held up in a slanting way, so that its wide end is exposed to the candler. As it is held with its narrow end between the thumb and the first two fingers, it should be turned quickly to left and right. This movement brings the yolk closer to the shell, and allows the observer to have a better view of the contents inside. Ideally, the observer should look at the egg with the candler right behind it. Though this process generally does not harm the embryo, exposure of the egg for long periods to the heat of the light should be avoided.

Interpreting the Observations

When the egg is placed in front of the candler, a distinct air sac can be seen at the wide end of the egg. The rest of the egg has the egg white with the yolk appearing as a dark circular area within the colorless fluid. If an egg that is over a week old looks like this, it means that it has not been fertilized.

After about 5 days, if fine red lines are seen in the egg, it means that the egg is fertile. These red lines are the blood vessels radiating out of a dark red spot which is the embryo. As the egg develops, the dark spot grows and finally only a dark mass and the air space are seen in an egg. At this stage the egg looks opaque. However, an opaque egg could also mean that it has rotten or that the embryo has died. In either case, it would be good to leave the egg inside the incubator for further observation. In case the opaque egg has a large air space, it is a sure sign that the embryo inside the egg has died. The increase in the size of the air space is due to the gases that are released as the embryo degenerates.

In certain cases, even after a week since it was laid, an egg might look clear albeit with an off-center air space that is larger than that in a freshly laid egg. This indicates that the egg never got fertilized and nothing developed inside it.

In case an egg looks opaque don't throw it away, observe the air space. If the air space in an opaque egg increases in size and appears as a large off-center space, then it means that the embryo has died. This egg can be removed from the incubator. However, once you are sure that the egg is dead or rotten, it is advisable to remove it to prevent contamination of the other healthy eggs.