Not all of the eggs in a group of duck eggs are fertile, and not all of the eggs live through the entire incubation process. Determining which eggs are infertile or dead is an important step in incubating the eggs. Dead or infertile eggs can grow mould or bacteria, which may spread to the other eggs. There's also, obviously, no need to incubate eggs that will never hatch. Luckily, there is a simple way to determine whether the ducklings in the eggs are dead or alive. This method is known as candling.
Hold each egg up to a bright light. There are several methods of candling. Some people use a box with a light bulb in it. A hole is cut in the box, and a direct light shines out. Others simply cup their hands around a flashlight, concentrating the light into the egg. Either way, the room where you candle should be dark, and the light should be both bright and direct enough that you can easily see into the interior of the egg through the shell.
Candle each egg for the first time at seven or eight days after the start of incubation. Although you can candle the eggs every day if you want, candling them at seven, 16, and 26 days should give you a good idea about the state of the ducklings.
Look for a bundle of veins in the egg at the seventh or eighth day when you candle. These veins mean that the duckling is living and growing. If the egg is clear, either the duckling has died early, or the egg was never fertile to begin with. If there is a red ring in the egg, and no visible veins, the duckling has died.
Look for any staining in the egg on the seventh day. This may be a spot of either red or black. If there are no veins present, this staining indicates a deceased duckling.
Candle the egg again between 10 and 16 days after the start of incubation. Again, look for a mass of veins in the midst of the dark part of the egg. This is a growing embryo. An egg that is simply dark, with no veins, may be dead. However, in this case, give the eggs a little more time. A dark spot can also simply indicate a growing duckling.
Look for movement in the egg. If you see the embryo moving, it's alive.
Look again for a red ring, or a black or red stain. These indicate dead eggs.
Candle the eggs at about the 26th day after the start of incubation, just before the eggs hatch. You should see a large dark mass, with a smaller light spot at one side of the egg. This is the duckling. You may also see a small beak shape in the light coloured spot. This is a sign that the duck is alive.
Look at the colour of the inside of the egg each time you candle. A vivid red or orange generally indicates a healthy egg. A duller yellow colour may mean the egg is infertile or dead.