These birds have no means to know the difference, and will feast on food they were never meant to eat. Their body will process it badly (without them even knowing it), causing improper excretion poising the waters, instead of continuing with the circle of life, where their natural 'poop' becomes part of the chain. A foreign substance has broken this perfect system.
I could take it one step further to say processed food manufacturers have done the same as this simple man feeding bread to the ducks. We are the ducks not knowing how badly we are processing the foods given to us, and told on the packaging how healthy it is. Only that diabetes, obesity, heart-attacks are now common. This is a result from the food we are eating. Not over-eating. Not not enough exercise. Just simple what we are eating.
I'm reading Gary Taubes : Why we get fat > more here
Taken from this blog ... I like feeding the ducks, papa was helping me breaking up the bread, some ducks wanted my starbucks muffin but I eat that all by myself.