In reply to Tom Valentine:
Red Kites are carrion birds and generally won't go for anything that will fight back, worms are probably about as adventurous as they get. They've got small talons that aren't well adapted for predating live animals.
Other birds will still mob them and the smaller ones will still see it as a threat hence the reason they bugger off. I don't think they can distinguish between them and other predators such as Sparrowhawks, Peregrines etc.