No, this scenario seems backward. First, usually, not always, but usually a male swan will not sit on the nest for any significant amount of time. The males usually sit by the nest, not on top of the eggs. Males are the ones who usually chase the juvenile swans, not the female. Finally, even though there may be eggs, mating continues until all eggs are laid, so this may actually be mating behavior you are observing.

Now, there are also other explanations. The mother swan that is nesting may be a new swan or a juvenile that the male has mated and the one they are chasing is the actual first mate. Sometimes, male swans will keep two mates and two nests within the area. Yes, it is unusual and rare, but can occur. Or, the swan that keeps coming back to the pond is a young male which can cause the commotion. Otherwise, we have no other explanation for the behavior you are describing. Please let us know how this situation progresses.The Regal Swan