Swan parents will discard bad or non-fertile eggs in the water. The last thing the parents want is to have a rotten egg explode in the nest. So, they will roll the eggs out of the nest and dump them in the water. Usually, predators such as raccoons, turtles, fish, etc., will eat the remains once the egg explodes. By dumping it into the water, there is less chance of ants and other pests near the nest.

The best thing to do with the egg is either leave it alone or toss it further into the pond. DO NOT handle it too much or you will be wearing a very nasty substance. Even if there was a chance for viability, once an egg is subjected to large amounts of water, the embryo will drown. So, it is very doubtful that the egg is viable.

Lastly, the strong fishy smell is due to at least 2 reasons: When the mother goes out into the pond to feed and bathe and then sits back on the eggs, she produces heat and humidity which can produce this smell. This heat and humidity is what helps the eggs hatch. Finally, the fishy smell could mean that the egg was in the water too long and picked-up the smell of the pond. The Regal Swan