feeding swans

I regularly see a pair of swans on my local canal, who seem quite used to humans. They seem to be travelling further along the canal in winter than they did in summer, I guess that food is scarcer. Can I help them by feeding them something? They don't seem interested in the sunflower seeds that I have for the garden birds but do seem interested in lunging at my hands! I was thinking that they might like some grass clippings from the lawn. Any advice would be much appreciated.