Smelly old bird food!

Hi there, I've been feeding the birds over winter on a patch of garden covered in stones. The old food has gone in between all the stones and has become rather smelly! Does anyone have any suggestions as to what I can use to put down and clean that won't harm the birds. I've since moved all the feeders and bird tables off the stones and onto the flags so all old food can be cleaned away easily but because all the stones are in a part of the garden which is raised and has side flags, unfortunately it's more like a very low swimming pool so it can't just be rinsed away. Many thanks in anticipation, the weather is getting warm now so it would be nice to sit out in a clean air smelling environment! lol