Bread is filling and high in carbs, but has little nutritional value. When ducks eat bread, it tastes good and it also encourages them to fill up on it, which can lead to protein deficiency iirc. Also ducks with diets low in certain vitamins develop a condition known as angel wing, which renders the wings unusable due to excessive feather growth (too heavy for young ducks' wings; developing wings are bent outwards due to weight).