Candling Eggs

Posted by Susan Lenz on


the step by step guide on how to candle your eggs and check fertility

Candling eggs is a great way of checking on the progress of your little babies developing inside the eggs. There are many reasons why you should candle your eggs. Lets take a look at how your incubating eggs develop: 

Breeders use candling to learn which of their chickens' eggs are fertile and will hatch into baby chicks. Candling can also be used to tell if a fertilized egg has stopped developing. The candling process works by illuminating the interior of an egg so you are able to see what is inside the shell. This article will show you the correct procedure for candling an egg.

Pictured above: top - how to correctly hold an egg for candling

middle: a fertilized egg being candled

bottom: the development of a growing embryo inside the shell

Understand why you need to candle your eggs. When you are hatching eggs at home, it is good practice to keep track of how the eggs are developing. However, this can be very difficult (if not impossible) without the use of candling. Candling involves shining a bright light into the egg, allowing you to see its contents and check whether it is developing properly.

  • When you are hatching eggs at home, you will very rarely get a 100% hatch rate. Some eggs will not be fertile to begin with (these are called "yolkers") while others will stop developing at some point during the incubation process (these are known as "quitters").
  • It is important that you are able to identify and remove these yolkers and quitters during the incubation process, otherwise they can begin to rot and eventually burst inside the incubator, contaminating the other eggs with bacteria and creating a very bad smell 

Use the correct candling equipment. Candling equipment doesn't need to be very fancy or specific -- in fact, in the old days it used to be done with the flame from a candle (hence the name). The main requirement is a bright light (the brighter the better) with an opening smaller than the diameter of the eggs you intend to candle. You will need to conduct the candling in a very dark room in order to see inside the egg.

You can buy specific egg candling devices in our retail store. These usually look like small flashlights, or pens, which are powered either by batteries or a plug-in cord.

  • Alternatively, you can take a very bright flashlight and cover the opening with a piece of cardboard with a hole (1 inch in diameter) in the middle
  • A more high-tech, expensive option for candling eggs is known as an Ovascope. This has a rotating stand on which you place the egg. The egg is then covered by a hood which blocks out any ambient light. You can then view the egg through an eyepiece, which magnifies the egg slightly for easier inspection.

Follow an appropriate candling schedule. You should candle your eggs before you even put them in the incubator. You probably won't be able to see anything, let alone differentiate between good and bad eggs, but it will give you an indication of what an undeveloped egg looks like, which can be useful for comparison later.

It may also be helpful to look out for any tiny cracks which are not visible to the naked eye. Cracked eggs are more susceptible to harmful bacteria getting inside and affecting the development of the embryo. If you find an egg with a crack, do not discard it just yet, but make sure to take note of the crack and check the egg's progress later.

Although some people will candle their eggs every day while they are incubating, it is a good idea to wait until about day seven. There are two reasons for this.

Number one: Eggs are temperature sensitive and constantly moving them in and out of the incubator could negatively affect their development, especially at this early stage.

Number two: Before day seven the eggs will not have developed very much and it will be difficult to distinguish between good and bad eggs.

After the candling on day seven you should leave the eggs alone til about day fourteen. At this point, you will be able to double check any eggs that you were uncertain about the first time and discard them if there are still no signs of development.

You should refrain from candling after day sixteen or seventeen, as the eggs should not be moved or even turned in the days leading up to the hatching. In addition, the embryos will have developed so much by this stage that they will fill the inside of the egg, so you will be able to see very little.


Hold the egg above the light. Set up your candling equipment in a dark room within close proximity to the incubator. Select an egg from the incubator and hold it above the light. The correct way to do this is as follows:

  • Place the larger end of the egg (where the air sac is) directly against the light. Hold the egg near the top, between your thumb and forefinger. Tilt the egg slightly to one side and rotate until you get the best view.
  • As you work, you should mark each egg with a number and take notes on your findings. That way, you can compare the results of your first candling with the results of your second candling.
  • Try to work quickly, but not so fast that you risk dropping the egg. As long as the eggs are returned to the incubator within twenty minutes to half an hour, there is no risk of the candling process affecting their development. A mother hen will frequently leave her eggs for short periods of time while she is incubating them.
  • Be aware that it will be more difficult to candle brown or speckled eggs as the dark shells do not become as transparent under the light.

Look for signs that the egg is a winner. A winner is an egg with a successfully developing embryo. You can tell if an egg is a winner using the following signs:

  • There will be a visible network of blood vessels spreading from the center of the egg outwards.
  • With a weaker candler, you might just be able to make out the clear bottom half of the egg (where the air sac is) and the darker top half of the egg (where the embryo is developing).
  • With a good candler, you might be able to see the dark outline of the embryo at the center of the network of blood vessels. You are most likely to see the embryo's eyes, which are the darkest spots inside the egg.
  • If you're lucky, you might see the embryo moving!

Look for signs that the egg is a quitter. A quitter is an embryo which has stopped developing at some point during incubation, for one reason or another. Some quit due to poorly maintained temperatures or humidity, some are contaminated by bacteria, while others simply have bad genes. There are lots of reasons embryos stop developing.

  • The main indication that an egg is a quitter is the development of a blood ring. A blood ring looks like a well-defined red circle, which is visible on the inside of the shell. It forms when the embryo dies and the blood vessels supporting it pull away from the center and rest against the shell.
  • Other indications that an egg is a quitter include the development of blood spots or blood streaks inside the egg. However, these dark patches can be difficulty to distinguish from a healthy embryo at this early stage.
  • If you are 100% certain that the egg is a quitter (the appearance of a blood ring is a very definite sign) then you should discard the egg immediately to prevent it from turning bad and exploding inside the incubator.

Look for signs that the egg is a yolker. A yolker is an egg that was never fertilized and has no chance of developing an embryo. You can tell if an egg is a yolker using the following signs:

  • The egg looks the exact same as it did when you first candled the eggs before putting them in the incubator.
  • The inside of the egg looks fairly clear, with no visible dark spots, blood vessels or blood rings.

If you are unsure, leave the eggs alone. If you think you might have identified a yolker or a quitter, but are not 100% sure, do not discard them just yet. If you do, you run the risk of throwing away healthy eggs.

  • Just make a note of which eggs have a question mark over them, then place them back in the incubator. It is always worth giving them another chance.
  • Check the questionable eggs again on day fourteen. If there are still no obvious signs of development or if a blood ring has finally formed, you can discard them.

Eventually, the chick will fill almost all of the space inside the egg and the air sack will increase further in size. When the chick finally hatches, it will break through the inner membrane into the air sack to take its first breaths of air. It will crack / push through the shell with its egg tooth (attached to the end of the beak). This allows further oxygen into the air sack so the chick can continue to breathe.

Over the final 24 hours, the remainder of the yolk sack will be absorbed, this will give the chick enough energy to turn inside the shell and slowly break from the shell as it goes. It will then use its feet to push itself out of the shell.

There is little point in candling eggs during the final few days other than to check for the air sack size because you will not see much. The chick almost fills the shell.

If you assist a chick in hatching, you take the risk of removing the shell too soon, before the yolk sack has been absorbed and before the navel has healed over where the allantois was attached. High humidity in the incubator will stop the membrane from drying out, keeping it soft and easy for the chick to break out. Humidity for the last three days should be 60% - higher and you risk the chance that the chicks can drown in the shells, lower and the shells will not be soft enough for the chicks to break out.

Chicks do not need food for the first 24-48 hours after hatching thanks to the energy they get from the yolk sack, and this is why most people recommend that you leave the chicks in the incubator for this period to dry completely and also allow the humidity to remain consistent for the remaining chicks to hatch.

for more information on hatching and brooding, we use and recommend this amazing book: HATCHING AND BROODING 

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