I think that bread is considered to be harmful to ducks because of the risk of crop impaction. While I agree that I wouldn't give your birds excessive amounts (because I myself don't feed mine too much to be on the safe side; I limit their intake), my grandpa raised a few Pekin ducklings years ago, and since he didn't buy feed to give them, he sustained them on bread and milk as babies, not to mention that they got spaghetti with sauce as a regular part of their diet once they matured. One actually lived to be 18 years old! However, even though their lifespan was extensive, those weren't exactly the ideal, healthy eating habits that I would encourage. I will say, though, my ducks love a little bit of carbs (within reason, of course) here and there.
-Alexandra33

-Alexandra33