Contrary to popular belief, ducks should not be fed bread as it can be dangerous for their health. There are lots of images in popular culture that show kids and parents, taking a stroll in the park and feed ducks together. In almost all cases, the food they are throwing them are breadcrumbs, but this should not be encouraged.
To help you get a clearer image, bread can have the safe effect of ducks as candy on kids. Having a diet based only on this will lead to malnutrition, weight gain, and pre-mature death eventually. Since feeding ducks bread will lead to weight-gains, they will use their ability to be agile and defend themselves from predators. They will also become dependent on this type of food and will rely on humans feeding them, instead of foraging for themselves.
Another big issue is that throwing bread in ponds and lakes is not great for environments. Once the ducklings are done eating, all the unconsumed food will stay in the water and rot there. This will also disrupt the eco-system of the water and the duck’s natural habitat. But no one is saying that you should not feed any ducks ever again.
The main idea is to make more documented choices and feed them something that is closer to their needs.
For instance, you can feed them grapes (cut in half), frozen peas or corn (at room temperature), cracked corn, oats and even birdseed. But the best option might still be the one in which you don’t feed ducks, as they will know how to find food for themselves and do not need human interventions in their habitat.