There is a sticky about treats here on the Duck Forum, and that is helpful.
Per Storey`s Guide to Raising Ducks, I put a tad of chick sized grit in their crumbles from day one. Just a light sprinkle, barely a teaspoon in a six inch chick feeder, worked in. They can digest crumbles without grit, but other foods will require grit for them to be able to digest and not have serious digestive trouble (impaction is really scary). So if they haven't had grit, I would wait on the other foods.
At about three or four days old I started adding chopped up rolled oats (not instant) to their feed because the protein was higher than the 20% recommended in Storey's Guide. Oatmeal is low protein, high fiber. My ducklings used to pick the oatmeal and eat it before they would eat the crumbles. Just dry oatmeal, and after several days the ducklings had grown so much I did not need to grind down the oatmeal for them.
I started when they were maybe 10 days old, with very finely shredded red romaine lettuce, dandelion greens, cress, and chard. I served them in a shallow tray with half an inch of water to make it more fun and easier to swallow. I also served salad right before it was time to clean and replace bedding in the brooder
After two or three weeks, I added just a few peas for treats (with eleven runners, that started at half a cup and is now up to about a cup and a half for everybody, once or twice a day, often skipping a day.
There was a recent post that reminded people that whenever you feed snacks, that affects the nutritional balance of their diet. So if you feed proportionally many low-niacin snacks, you need to find a way to boost the niacin. Same with protein. If your feed is about 20%, low protein snacks will lower that, which is not a good idea the first two weeks, but becomes less of an issue the older they get, until they are laying, or in cold weather.
That's about what I can offer.
Enjoy the little ones - they grow up quickly!
