The hen won't start sitting on a nest (incubating the eggs) until she has laid all of the eggs. This ensures that they all hatch within a few hours of each other. Twelve eggs would be a normal sized clutch. However, she wouldn't wait several days before starting to incubate the eggs. So I am thinking of two scenarios. One, she started incubating the eggs and something happened to her. Once the eggs start to be incubated they cannot go very long without warmth. Certainly four days without being incubated would mean that the eggs are no longer viable. Another scenario could be that this is a nest that several female ducks contributed eggs to in the hopes that another female would incubate the eggs. But sometimes in these cases, the hen that built the nest decides to abandon the nest and the eggs just go to waste. In Illinois, mallard eggs are protected by state and federal laws. I will send you the number of the Illinois Department of Natural Resources district wildlife biologist that covers your area. He will be able to provide you with advice about what to do with the nest.
