I am hoping you can help my wife and I better understand swan-rat interactions. The exact species of either is not important, but we are leaning towards mute swans and Norway rats. That said, if you have better or more exhaustive data on other species of swan or rat, that will be welcomed.
Full disclosure, we have not reached out to the rat community, as we feel at this time, they are too busy trying to cure cancer/etc.
Very interesting question. Depending on the size of the rat, there really should not be a problem unless the rat tries to enter the nest and eat the eggs. The major issue is to feed the swans in the water approximately 3 feet from the bank. If you leave food onshore, it will attract rats, ants and other pests.
Now, having said all of this, if the rat is not a rat, but rather a mink or such a larger animal, it can attack and kill the cygnets (baby swans). If the rat were to swim further into the water, the swans would probably try to drown it. We hope this info is of benefit. The Regal Swan