Not in the sense of it being toxic for them or anything, but it doesn't contain the right nutritional value ducks need and can fill a duck's stomach to where they don't have room for other foods they would naturally eat that are healthy for them and can lead to malnutrition.
Next time you plan on going to the park or some place where there's ducks you like to feed, here are some healthier alternatives to bread you can feed them:
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@MellyMel22 They can digest it, it just doesn't really do much for them in terms of nutrients. Though stale and bad bread definitely isn't good for them and can make them sick with diarrhea. Unfortunately, people feeding ducks spoiled bread seems pretty common. Not sure what people who do that are thinking. If it's not good for humans, what makes them think it's gonna be good for ducks? 🙄