I have 4 12-ish week old Cayuga duck hens that are picky eaters. 
 
They were always given  brewer's yeast w/ garlic and probiotic powder as ducklings.
 
As hatchlings they loved 
manna pro  game bird starter mixed with nonfat yogurt. But since I didn't have a local source for it  when I finished that bag I  tried 
all of the different duck chows at 
TSC.  They would just pick at them and made a wet mess. Purina anything was their least favorite.  Dumor they ate OK for about 2 weeks and then they started picking at it.
 
I tried Farmer's friend chicken kibble mixed per instructions with old fashioned oats.  They loved that mix. So I'd use it when I worried they weren't eating enough of the  premixed duck chows.
 
I started them on scratch & peck naturally free feed and lots of supplementary grit.  They would pick at it.  
 
I tried water fermenting feed. They would pick at it.
 
I tried  steaming veggie waste stems (collards, chard etc.), mixing in the steamed veggie stems and using the steaming liquid to moisten the feed.  They'd pick out the steamed bits and eat a bit more of the duck chow.
 
I tried mixing in non-fat yogurt with their feed. Bingo!  Feeder clean within an hour. Since I made that discovery all of their feedings are moistened with yogurt in a 1 to 1 ratio.
 
(For my next experiment I'm going to try  yogurt +  any 
TSC duck chow to see if they'll eat it. But i still have 10-ish pounds left to feed before I start that experiment.)
 
 
When they were smaller and growing like crazy, I'd steam and mash potatoes for them just to keep myself from worrying about them being hungry. 
 
I have always made sure they had a dish of chopped greens to gobble. As ducklings any variety of  cabbage was their favorite.  Now they'll eat beet greens ,turnip greens, fennel fronds, green plants and herbs I clip from the yard, etc. 
 
Now that they're older and I can see that they're growing into beautiful hens, my gut tells me that the birds that are good foragers do a good job of finding worms/bugs/greens/herbs as long as plenty of forage area and forage time is available.  They're so efficient at it that we call them the four ducks of the apocalypse... 
Nothing escapes them in foraging formation.
 
I don't mind mixing in yogurt with their feed.  Their poop smells less and it is easier to clean up.   Their home smells less ducky, too, because they aren't wasting the food.
 
And i don't have to worry about them being hungry.
 
They still get a dabbling dish of greens, too. And steamed potatoes for treats a few times a week because I enjoy treating them. 
