Here's one of the many articles explaining :
Many birders were first introduced to the joys of wildlife by feeding ducks bread at a local park or pond. While we may have thought this was an environmentally responsible and cute way to dispose of old or stale scraps, bread is bad for ducks because it's unhealthy and potentially dangerous for birds and other wildlife.
Bread and similar products such as crackers, rolls, chips, crisps, donuts, crusts, cereal, and popcorn have little nutritional value for ducks, geese, swans, and other birds. Bread is the equivalent of junk food for birds. Just like humans would suffer from a diet of nothing but candy and sugar, too much bread can lead ducks to obesity, malnutrition, and many other problems.
Ducks need a balanced diet full of amino acids and vitamins—which they get from vegetables, whole grains, and berries—not highly processed baked goods.
Duckling Malnutrition: Ducklings require a varied diet and plenty of natural plants and insect proteins to mature properly. If ducks are regularly fed bread, ducklings will not receive adequate nutrition for proper growth and development.
Since ducks eagerly seek an easy food source, such as human handouts, ducklings will not learn to forage for natural foods.
Overcrowding: Where an easy food source is abundant, ducks and other waterfowl will lay more eggs, and the pond or lake will quickly become overcrowded. This makes it more difficult for the birds to seek healthier food sources and increases the likelihood of territorial aggression. In crowded areas, predators can also thrive and impact other bird populations, and diseases can quickly spread through large flocks.
Pollution: When too much bread is offered to ducks, not all of it will be eaten. Soggy, uneaten bread is an eyesore. Rotting bread and increased bird defecation can create noxious odors and lead to significant algae growth that can clog waterways and crowd out more desirable plants.5 This concentrates the pollution and can eventually eradicate fish, amphibians, crustaceans, and other life in the vicinity, making good food sources even scarcer.
Diseases: Feeding ducks bread can increase the spread of infections in two ways. First, a carbohydrate-rich diet leads to greater defecation, and bird feces easily harbor bacteria responsible for numerous diseases, including avian botulism. Second, moldy bread can cause aspergillosis, a fatal lung infection that can decimate entire duck and waterfowl flocks.
