This is the time of the year that swans mate. So, they are becoming very defensive of their habitat (a normal attribute). They will chase you and everything else away from their habitat, especially if they feel there is a threat. Humans of any kind are seen as a threat around eggs and cygnets. White birds are seen as a rival, very similar to another swan in the area. Some swans can get along with all ducks and geese and some swans cannot get along with anything or certain inhabitants of the pond. What you are describing sounds like the birds are just acting normally and for some reason may just not be too hospitable to the muscovys. Is there anything that may have occurred, such as the muscovy eating swan food or getting too close to their nesting, bathing or normal hanging out area? If so, this could have been the trigger for the swans' behavior. The Regal Swan