Lettuce: water. That's honestly why we use lettuce in human food. Well that and the crunch.
Seeds: protien/fat
Peas: some potassium, iron, vitamin B6, and vitamin C
Oats: good carb/fiber/protein balance
Source: Chef that isn't fat. I'm not skinny either, but I know enough about the nutritional content of the foods I make to graze without getting more than a little chub
Bread is a good and easy source of carbs. Even if it has other nutrients, the duck is going to be consuming far more carbs than it should. While the other things have carbs, they aren't nearly as bio-available. It's like drinking juice that's had a bunch of sugar added and thinking it's healthy because it's juice. Carbs are the thing that'll make you fat, regardless of what other nutrients come with it.