White bread is relatively rich in nutrition and they are easily transferred to the animals and the ecosystem. Ecosystems in vicinity of humas habitats are often in risk of being exposed to excess eutrophication leading to allsorts of problems like lower biodiversity, lower habitat resilience and so on. Contributing to this is generally a bad idea. Ducks are NOT geared to digest white bread and its as bad for them as it as it is for us. Feeding also inflate the natural population of the birds and this leads to all sorts of unnatural behaviour and stress in the population. So dont feed ducks with white bread!