I've used ferret droppings to ward off mice from my storage locker.  Two
years ago it worked great.  Not one mouse in the place.  This year it
didn't.  Maybe I didn't place them in the right corners.  Or maybe I
should have used fresher ones.  But it's natural, harmless, and I have
plenty of it to get rid of anyway. :-)
 
If you do have mice, get rid of them, though.  They can transmit
leptospirosis, which mustelids are very susceptible to.  It's transmissible
through urine, so even if the mouse always hides when the ferrets are
anywhere near, they can still catch it from the evidence of the mouse's
passage.
 
E.S.
 
[Posted in FML issue 0669]