Feed ducks grapes cut in half. Bread is actually bad for them.
Adam Rifkin stashed this in Actual Advice Mallard
Why not feed bread to ducks?
"Eating bread will fill them up. Then they don't eat the food they really need. The most important thing that gets out of whack is the Calcium:phosphorous ratio. It gives them brittle bones and also creates weak eggs. I'm not convinced grapes are really the best either, but at least it wouldn't be as filling as bread."
Same reason bread's not all that great for people: it's not very nutrient dense. Also the GI tract is not a very big fan of gluten.