I was by the river today, and observed the usual gang of fat-ducks being fed bread. One lady got bit (much to her surprise and my amusement) whilst trying to hand-feed.
Then the mob were spooked by something and all went for the river. I noticed a small hawk, but given that it was much smaller than the ducks I didn't see it as a threat. OK maybe the ducks took a cue from the pigeons, but I wouldn't expect a small hawk to go for a pigeon either.