For whatever reason, nature knows that something is wrong or that the pen cannot successfully raise all of the cygnets and so some eggs may be fertile, but do not survive. Usually, dead cygnets in the egg means that the mother bird cannot wait until the remaining cygnets hatch or the live ones will perish. In nature, mothers are wired to hurry up and get the surviving young out into the world so that predators do not take the whole family.

If you open the eggs unnaturally, the cygnet will more than likely still die as this would be like removing a child from the womb without being ready to be born. There is a set time that the cygnets are gaining nourishment from the egg and they have an attachment to their bodies from the egg (think of an umbilical cord in humans) that allows them to gain nourishment. If the bird is separated from this structure it means death. This is why eggs should not be turned or moved too quickly during the incubation process and the mother birds knows exactly which direction and how far to rotate the eggs. The Regal Swan