Egg candling helps chicken farmers distinguish between eggs that are yolkers, quitters and winners. The farmer uses a bright light to see through the shell and determine if there is a developing chick inside. The process requires a bright light and a darkened room. A flashlight, a homemade egg-candling device or a commercially available candler provide the light.
Holding a winner, or an egg in which a chick is developing, to the light reveals the growing embryo. It is possible to see a dark spot at first, but as the embryo continues to grow, the observer might see developing blood vessels or a moving chick inside the egg. These eggs are developing properly and are returned to the incubator.
A yolker is an unfertilized egg. Because there is no growing embryo to consume the yolk, the heat of the incubator will cause the contents of this egg to rot, creating a bad smell in the incubator.
Quitters are fertilized eggs with embryos that stop growing before they hatch. While there are many causes for this, embryos in eggs that get too cold or too warm often stop developing. Since the embryos are no longer alive, these eggs also begin to rot and smell.