When they first hatch, wild baby ducks can live off leftover egg yolk stored inside them for up to three days as they travel with their mothers to a food source. For the next few weeks, they eat fatty and protein-rich foods like insects, crustaceans, larva, snails and worms.
Ducks mature very rapidly, and after only two or three weeks the ducklings will already look much like an adult. At this time, their diet also begins to become more like that of an adult, as some species begin incorporating more plants into their diet.
The diet of adult ducks varies depending on habitat and also from species to species. For instance, wood ducks tend to consume a large amount of fruit and nuts because they live in forests, while a mergansers' sharp, toothed bill allows it to subsist primarily on fish.
Ducks that live at the edge of the water usually eat insects, amphibians and aquatic plants, and ducks that prefer open water usually dive down deep to catch fish and crustaceans. Ducks that live in open fields or parks primarily eat grass, grains and seeds.
Although people often feed bread, crackers and other food items to wild ducks, this can actually cause problems for the ducks. These foods provide little or no nutritional value to ducks and can actually make them obese or lead to illnesses and even death.