A predator could have gotten the eggs such as a human, fox, dog, cat, bird, etc. There is also the possibility that the swans knew something was wrong with the eggs and rolled them from the nest into the pond. If this occurred, turtles, fish and other water life would have eaten the eggs. If the birds were acting like nothing happened, then they had to know something was wrong with the eggs and did not bother trying to defend them or even now care that they are missing. The Regal Swan