When ducklings swim in a line, they make use of the principle of hydrodynamics. By swimming in a line, the ducklings can take advantage of the 'waves' created by their mother. The mother duck leads the way, creating a current that reduces the resistance for the ducklings swimming behind her. By the time it comes to the third duckling in the line swims, the resistance is virtually eliminated, allowing the ducklings to move forward effortlessly with the flow of the water.