Regarding last year's cygnet, yes, we have seen parents not willing to help something weak as this totally goes against the theory of "survival of the fittest". If young wildlife cannot keep up with the parents or the rest of the family, it could mean that it is sick or something is wrong with it which would not be something wanting in nature to pass on through genetics. So, nature wires wildlife parents to only care for those young that are strong and survivable. Secondly, predators can be attracted to and follow the rest of the family if something weak keeps hanging around. The male parent may have intentionally killed the bird to remove it from the family, relieve it of its misery and prevent predators from taking it.

As for this year, the cygnet was in the same situation. It may have hatched too soon and the parents did not want to abandon the rest of the eggs and possible more hatchlings for one bird. The egg could have been opened due to accidental or intentional rolling from the nest by the parents (they know when something is not good and yolk is a sign that it may not have been fertile or was not forming as fast as the other eggs) or a predator.

If the laying of the eggs occurred too soon, then cold weather or other environmental causes may be the reason that the first egg hatched too soon, and the other eggs have not. If the parents are still sitting on the eggs, it usually means that again, they know when something is viable and they will sit until proven otherwise. So, probably, some of the eggs are fertile and viable. If one or a few of the eggs hatch, this time, the parents may abandon the rest of the eggs since they have been sitting for a long period this year. This is just nature in motion. The Regal Swan