There could be any number of reasons:

1. Predator in the area has visited the nest and has scared the parents.

2. All eggs are infertile. How old are the birds? If they are only 2-3 years of age, this may be their first clutch and they are in the "practice" mode.

3. Something happened to the eggs to make them infertile, i.e., too much heat and then cold snap, etc., or someone shaking the eggs.

4. Parents are stressed about sitting for such a long time on the nest due to heat fatigue, hunger, etc.

5. Parents were not cut out to be parents. We have seen swans lay eggs, hatch out the cygnets and abandon the cygnets because the parents either were too young to know what to do or did not want to bother with their offspring.

We hope this info is of benefit. The Regal Swan