Feeding a wild duck?

where i work we have a group of ducks that come to breed we r not allowed to feed them however last year a single drake came to the flat roof of our building in early feb i couldnt help but feed him he stayed for nearly two months coming every morning for his breakfast of tuna sanswiches and bread, then left i guess to breed. this morning he came back a yr exactly and was quaking at the window where i used to feed him i gave him some bread but have now mixed up ready break, porriage and bread to give him in the morn am i doing the right thing ? or am i harming him ?