I was watching a British documentary about the sometimes aggressive, and downright hostile nature of these birds. It's a particular breed of swan (I can't remember the name) that are the most ferocious.
Of course, they will attack you if they think you are threat such as standing to close to their nesting site. What is ironic is that the people who take care of these birds know about their violent behavior. Swans will violently attack anyone if they get close enough. Like the idiot worker in the documentary who went right up to the nest (which by the way had eggs) just to "make sure everything was all right." I don't know if it was the male or female species that assaulted him.
Also, there's a couple who is suing some zoo or sanctuary because the swans they take care of attacked and killed a man while he was out in his boat. The wife of the dead man states that the people who take care of these birds knew they were dangerous and didn't warn the couple. The swan attacked the boat, tipping it over and the man drowned. Cheers!!