Realise you left this post a while ago, but I've just seen it. I know what you mean about the mess.. but I don't think anyone clears it up.. I think just the rain washes it away. We seem to get enough rain for that..
I think it's nice to see the kids feeding the birds it's a fun free thing for them to do. I expect if it becomes too bad with getting attacked by the birds something will have to be done. For now I'm happy to just watch where I walk.