Why birds fly and duck swim?

Ducks are birds, and actually they can fly quite well. However, I believe your question is why can ducks swim when most other birds can't. The reason for this is evolution - ducks and other waterfowl like swans and loons have evolved into a niche that involves them eating directly from the water. Part of this evolution involved developing oil glands or preen glands on the base of their tails; waterfowl like ducks preen themselves constantly to spread this oil on their feathers to keep themselves waterproof so they can float on the water. Most non-waterfowl birds fly because they have evolved into a niche that involves them eating or hunting on the wing. However, I will note that not all birds can fly - penguins, ostriches, emus, rheas and kiwis all can't fly, but they are birds.