If you are unsure if an egg is dead or alive, return it to the incubator and check again in a day or so.
A challenging aspect of raising ducks is discovering a recently hatched egg is dead or infected. Not every duck egg goes on to hatch a healthy duckling — some embryos die in the eggs, or the eggs become rotten or infected when bacteria grow inside them. Knowing whether your duck egg has a live embryo inside is advantageous. Through a process called candling, you can look at your duck egg and likely determine whether a live embryo is inside.
Turn off all lights and light sources in the room in which you intend to candle. The candling process works only if it is done in a room that is entirely dark.
Hold a small flashlight above the top of the egg, so that the light is shining directly down on it. Hold your hand around the end of the flashlight so that all its light is directed down toward the egg and doesn’t come back up to your eyes.
Look for any veins within the egg, starting at the seventh day of incubation. Typically, the discovery of clear, distinct veins is a sign that the egg is alive. After the 12th day of incubation, you may notice a live embryo move when the light is directed toward the egg. On days 26 and 27, you may see the bill of the duck moving inside its air sac.
Note any rings or streaks of blood within the eggs. Such characteristics are usually indicative of dead embryos or embryos that never developed. Additionally, the absence of veins is usually indicative that the egg is dead. Infected eggs are often dark and blotchy inside. Remove any infected or dead eggs from the incubator.