When you are raising birds, either poultry such as chickens or turkeys or pet birds such as canaries or parrots, it's important to determine if their eggs are fertile. Female birds often lay eggs, whether they have been fertilised or not. Determining whether the eggs are fertile will allow you to carefully watch those eggs that are fertile and discard those that aren't, saving you from caring for or incubating eggs that will never hatch. One of the easiest methods for determining fertility is called candling.
Cut a small hole in a cardboard box of any size. Place a flashlight or other light in this box and turn it on, so that the light is directed through the hole. This will help to concentrate the light. Place this candling device in a darkened room. The dark will help you see the insides of the eggs.
Hold one of the bird eggs above the light. You can rest the egg on the hole in the box, in order to direct the light through the shell. Be careful not to drop or jostle the egg. Aside from these possible accidents, candling the bird eggs will not harm them. Most bird eggs can be candled between four and seven days after they are laid.
Observe the lit insides of the eggshell. What you see will help you determine whether the egg is fertile or not. Fertile eggs at this time will show a dark blotch near the middle of the egg, with many spiderlike veins protruding from it. This is the embryo of the living chick. Infertile eggs appear to have clear or blank insides. You may also see a dark shadow in an infertile egg. If the shadow has no blood vessels branching from it, it is simply the yolk of the infertile egg.
Return any eggs that you could not positively identify as fertile or infertile to the incubator or nest. Candle these eggs again between 14 and 16 days after they were laid. At this point, a live and fertile embryo will appear as a dark mass when held to the candling device. A few clear spaces will also be visible. These are air cells and should have blood vessels running through them. You may see the chick moving about, a clear indication that the embryo is alive. Dead or infertile eggs will appear clear, as before, or may have a dark ring running through them. A dark area with no blood vessels may also indicate a dead embryo.