Lots of people get enjoyment out of feeding wild birds and ducks, primarily with stale or uneaten white bread that might be left over. While the gesture is kind and you're in fact making them fat and happy, you're also probably not doing them any good in terms of their health.
By eating our food, ducks can end up with a health condition known as angel wing, which makes them unable to fly and dodge predators.
Birds aren't the only creatures that we tend to feed. We also seem to like feeding monkeys, which can lead to the dispersal of bacteria that can make them sick. Other creatures can actually abandon their own kind when humans interact with them, such as the recent case with a wild bison, and this will lead to their premature death.
The general rule of thumb is that feeding wild animals probably isn't a good idea. They've gotten as far as they have today by fending for themselves, and by helping them, you're making the animals more dependent on you.
Just leave the wild animals alone and eat your own food. It's for the greater good of both of you.