We put out two trays of food for the ground feeders which is eaten but when the tray is empty someone puts a load of small stones in the empty tray? Any ideas?

Very odd, if I remember correctly I read something about this before. It may be one of the Corvid family, unsure whether crow, rook, Raven or even Magpies. I may of course be talking rubbish. Off to have a Google and see what I can find? 

Found a video about crows that were bringing different objects to a little girls garden, so it could well be crows.