Feeding wild ducks isn’t usually a good idea. Even when the food is healthy, over-feeding tends to make ducks become dependent and can cause them to over-populate bodies of water. This leads to the overgrowth of algae and the mass death of fish, and can sometimes cause ducks and geese to become aggressive toward humans.
However, if you’re going to feed ducks anyway, please feed them healthy foods. Bread causes severe gas, diarrhea, liver disease, and bone deformities. Make sure you don’t accidentally cause more harm than good!