Ducks ca eat bread, but you should not give them bread. It’s really bad for duck. Of course, the diet of domestic and wild ducks is significantly different, but in that, and in another case the use of bread alone will lead to the fact that the bird can simply die!
Therefore, if we are talking about birds that are kept in captivity, it is important that their diet is balanced: a little bread, grass, corn, wheat, oats, beans and so on. This should be strictly monitored.
But if we are talking about wild ducks, then there is very little that depends on us, because such birds always seek for their own food. In Moscow, by the way, there are always wintering several thousand representatives of the duck family.
So, many of them simply will not eat the crumbs left in the pond, since they basically eat different invertebrates and fish. But even if the duck swims and eats this bread, believe me, nothing will happen to her! Perhaps she will even say thank you, as this will be a good addition to her dinner. But, again, just an addition!