The Diet Page
Here is the best feed formula that I have come across for Call Ducks.  The feed is prepared as a pellet and this allows the duck to eat all of the pellets and the individual items as one.  They can not pick through it and eat only certain grains.  Another benefit for the pellet is with a mash style feed, the ducks seem to waste a lot of it in their water dishes and it mash seems to stick to the ducks and makes them dirty.
One benefit I have noticed since I switched to this feed is the ducks absolutely love their feed.  They hardly touch their peanuts and other treats.  Every single duck on the farm loves this feed.  You will need to still add a little ration of oyster shells when it is breeding season as sometimes the ducks do not get enough calcium. 

Here is the feed formula that Bill Durbin of Hutchison, Kansas developed:

20% Chicken Breeder Ration

970 lbs Corn-Milo-Oats-Wheat (use a combination of 2 grains if possible)
100 lbs Dehydrated Alfalfa Meal
300 lbs Wheat midlings
200 lbs Meat Scraps
300 lbs Soybean meal
30 lbs   Di-Calcium
70 lbs   Calcium Carbonate
20 lbs   Salt
10 lbs   Vitamin Pre-mix
1-5 lbs  Trace Minerals (Follow Manufacturers directions)

Of course, you can not use meat scraps any longer so your mill will have to substitute another form of protein.  My mill uses more soybean meal and the ducks love it.

Taken from:
Sheraw, Dr. C. Darrel, The Call Duck Handbook 2003, 2 nd edition, pg 93.

In the winter I use this feed but add some corn or red wheat which provides heat and warmth during the digestive process for the cold Michigan winters.  Otherwise this feed has worked very well for our farm.
We do give treats to our calls and they include: minnows; peanuts; lettuce clippings; and pond fish food.