Feeding ideas in a nutshell:
You might start with feeding them commercial feed – layer if they are adults and grower if they are ducklings. Unless you can get special duck feed, you'll have to add extra niacin. They also need grit to “chew” their food (they don't have teeth!).
You can also feed plain grains, which are healthier (without all the chemicals, etc.), such as wheat, sorghum, corn, and oats.
Forage is probably the healthiest feed, though. If they are in a grassy pen, they will have fun munching on grass and scouting for bugs. If they have access to a pond or puddles, they will find waterbugs, tadpoles, worms, and other delicious goodies. They also love slugs.
Many people raise mealworms or earthworms for extra protein for their ducks. I have no experience there but would like to try it.
Lastly, you can feed them veggies and fruits. There are tons of things you can feed them: watermelons, bananas, sunflower seeds, tomatoes, peppers, pumpkin, cucumber, lettuce (my ducks' favorite), cabbage (another favorite), and more.
Here's a nice list of “treats” (though I like to feed them as a relatively large part of the diet):
https://www.backyardchickens.com/a/chicken-treat-chart-the-best-treats-for-backyard-chickens
Overall, this is a good duck diet: commercial feed, niacin, grit, grains, forage, worms, and veggies that you give them. You don't have to feed all of these components, however.
(Note that this isn't a complete coverage of duck feeding. You can try looking through the articles and forums on BYC for more information.)
~Hannah
