For starters, bread is essentially just junk food. It fills birds up, but it doesn't carry much nutritional value. That means pretty much the same thing it means for humans: baby birds are more likely to end up malnourished, without the plant nutrients they need to grow properly. A high-calorie, low-nutrient diet can cause waterfowl to develop a condition known as "angel wing", an incurable wing deformity that usually renders birds flightless. It's sad as heck, y'all. And then there's the fact that the constant availability of human-supplied junk food keeps ducklings from learning how to forage healthy food for themselves. Meanwhile, grown-up birds who gorge on carbs are more likely to get fat and sick.