I looked it up, and bread has about double the amount of carbs as cooked rice, which is probably what is making bread bad.

Probably a rice-only diet would also be bad for them, but nobody brings them tons and tons of rice, while everyone throws them a ton a bread.

Attention shoppers: we need a duck scientist. Duck scientist to the thread, please.

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).

I've heard that they eat more grains if the rice is uncooked and uncooked rice expands in their stomachs causing pain and other problems. Cooked rice is already expanded so they won't overeat