The species of duck defines if it is a flightless bird. Most domesticated ducks have lost their ability to fly due to the lack of need to fly to search for food and shelter. They have micro-evolved to be less dependent on their environment and more dependent on their caretakers for food and shelter. Many wild ducks, like the Mallard duck, can fly for long periods. Some wild ducks do lack the ability to fly, including three out of the four steamer duck species.