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What is Candling?

Mary McMahon
Updated Mar 05, 2024
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Candling is a technique which is used to study embryonic development in eggs, and to assess eggs for commercial viability. People have been practicing candling for centuries, as the name implies: originally, the only piece of technology used in candling was a candle, although today bright sources of artificial light are used. Candling is very easy, and it can be very interesting, even with plain store bought eggs.

When an egg is candled, it is taken into a dark room and placed in front of a very bright light source. The light source allows people to visualize the structures inside the egg, which may be brought into even sharper relief by gentle jostling which pushes the yolk up against the edge of the shell, making it even easier to see.

During the candling process, it is possible to see flaws in the egg, such as blood spots, and people can determine whether or not the egg has been fertilized. If the egg has been fertilized and it is being incubated, it is possible to follow the steps of embryonic development in the egg by candling it on a frequent basis. As the chicken develops inside the egg, a spiderweb of veins will slowly appear, and the shape of the chick will start to emerge, like a very slow-developing photograph.

When eggs are prepared for sale, they are often candled to ensure that they are good to eat. Eggs with flaws or signs of being rotten will be discarded, while healthy, fresh eggs will be packaged for sale. Since some people object to fertilized eggs, farmers with free range mixed broods may candle eggs to separate fertilized from unfertilized eggs for sale, and farmers may also collect fertilized eggs to hatch into new chickens.

Although candling is a lot more fun when the egg is fertile and an embryo is developing inside, it's pretty neat with unfertilized eggs, too. If you have a bright lamp or flashlight, some eggs, and a dark room, you can try out candling for yourself. It usually takes a few tries to get used to the experience and to understand what you are seeing when you candle, and you may find a visual guide helpful. Several organizations including the United States Department of Agriculture issue candling guides with clearly labeled photographs illustrating the structures inside eggs.

Frequently Asked Questions

What is candling and how is it used in egg inspection?

Candling is a method used to evaluate the quality and development of an embryo inside an egg. By shining a bright light through the shell in a dark room, one can observe the contents of the egg. This technique helps in determining the viability of the embryo, detecting blood spots, and identifying any developmental abnormalities or internal cracks. Candling is crucial in the poultry industry to ensure only fertile and healthy eggs are incubated.

Can candling be used on all types of eggs, and are there any limitations?

Candling can be used on most types of bird eggs, including chicken, duck, and quail eggs. However, it is most effective on eggs with lighter shells, as darker shells can make it difficult to see the contents clearly. The technique also has limitations in the later stages of incubation, as the developing embryo can obscure the view.

At what point during incubation should eggs be candled?

Eggs are typically candled at two key points during incubation: around days 7 to 10 and then again at day 18, just before "lockdown," when eggs are left undisturbed until hatching. The first candling checks for signs of development and embryo viability, while the second ensures proper growth and removes any non-viable eggs to prevent contamination.

Is candling a safe practice for the eggs and the developing embryos?

When performed correctly, candling is a safe practice that does not harm the eggs or the developing embryos. It's important to handle the eggs gently and minimize the time they are out of the incubator to avoid temperature fluctuations. Excessive handling or heat from the light source can be detrimental, so care must be taken.

What equipment is necessary for candling eggs, and is it expensive?

The primary tool for candling is a bright light source, often a specialized candling lamp. These lamps range from simple, inexpensive handheld devices to more sophisticated and costly models with adjustable intensity and focus. For hobbyists, a strong flashlight in a dark room can suffice, making candling an accessible technique for many.

Can candling determine the sex of the embryo within the egg?

Candling cannot determine the sex of the embryo within the egg. The technique is used to assess the development and health of the embryo but does not provide any information regarding its gender. Sexing chicks is a separate process that typically occurs after hatching, often through vent sexing or feather sexing, depending on the breed.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a AllThingsNature researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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