Swans are assholes. There was a family of swans living near my house a couple of years ago, and everytime someone walked near the pond he'd actually swim up to them, get out of the water and be all 'come at me bro'. He actually attacked a dog, I believe.
Or, when they don't have kids, it's just one or two of them, they'll come up to you and demand food. YOU. BREAD. NOW.