According to About.com, fish are one of the most common foods ducks eat. Ducks also typically feed on snails, worms, crustaceans, grasses, weeds, aquatic plants, small amphibians, insects, seeds, fruits and nuts. Ducks are omnivores, but their diets vary for several reasons.
Some foods are more available during certain seasons. For instance, insects are abundant during spring and summer. A duck's habitat makes a significant difference. Ducks living near water have greater access to fish than those with homes in fields. Feeding style affects a duck's diet, as well. Some ducks are divers and access plenty of fish and crustaceans. Others stay in shallow water and focus on plants and insects.