My shitbag dog (16 months) is obsessed by picking up random pieces of crap on walks and trying to eat them. Mainly feathers and bits of tissue/paper. Obviously I try and steer him away from stuff but you don't always see them pick stuff up. Today he bit me twice when I tried to get a feather off him. He's not very good at 'drop' - I know I need to work on this. Is there anything else I can do to stop the resource guarding?