This is not normal behavior between a male and female swan. However, this behavior would be considered within the realm of normal if both swans are of the same gender. This whole scenario seems to indicate that you have swans of the same gender. In swan species, males will mate with males and females will mate with females when opposite genders are unavailable.

Males are responsible for building the nest and will actually build nests together or will go off on their own to build a nest. Females will do the same and may even go as far as to lay infertile eggs. So, this is probably why you are seeing the now aggressive behavior. However, all should calm down in the next few weeks when mute swan nesting and mating season will cease and hormone levels drop.

Male swans will assist in watching the nest if the female lays eggs. He will sit by the nest while she bathes or feeds from the nest. Depending on species and individual swans, some males will even go as far as to sit on the eggs until the female returns to the nest. The Regal Swan