How to check the fertility of an Egg

1.   First Hold the egg above the light. Take your candling equipment in a dark room within close proximity to the incubator. Take an egg from the incubator and hold it above the light. The correct way to do this is allows:
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! [3]
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.
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. [1]
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. [5]
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