<<I've heard you can't break an egg
suckin' dog, but is it possible to break a duck killin' one? Any advice
besides get rid of him?>>

It's never a sure thing to break a dog of killing anything, but it can
often be done.  I think it really depends on the dog and why he is doing
the killing.

I have heard of several solutions to this.  Some of them I have tried
successfully in the past, and some not.  Countryside and Small Stock
Journal had one a few years ago that sounded promising, but I have never
tried it.  They advised "hot wiring" a dead chicken (in your case, duck)
and placing it in plain sight.  Apparently, this method even cured hunting
dogs of going after the birds!  Tying one of their kills around their neck
in a way they cannot rid themselves of it for 3 or 4 days is also a tried
and true method, particularly in hot weather.  It doesn't work if you have
other dogs more than willing to pull it off though!  I suppose a shock
collar could also be used for this, but it might turn him off to ever
working poultry, if that is a concern for you.

It seems like the first step might be to get complete "stop" control of the
dog first.  If you can drop him every time he gets in too close, he may
start to get the idea, unless he is really a confirmed killer.   We had
some problems about a month ago with our dog Jacob going after baby poultry
(although he never killed one, I was afraid he might).  Having already
taught him the command "GENTLE!" when he got too rough with the stock, I
just persisted with that command whenever he started to get rough.  He got
the idea within about three days.

If all else fails, and you don't want to get rid of the dog, confine the
birds.<g>  Just to keep track of our birds, we have them in moveable
stock-panelled pens and it works very well.  By moving them every day, you
don't have to clean up after them and they would be protected from the dog
if he won't leave them alone.  Unless your dog is one who runs hawks off,
 the ducks will be safer in confinement anyway.