A drone research team from Stanford University measured the forces involved in a bird's hovering and found that it created "double the lift during the downstroke [of the wings] so that the birds did not have to lift their weight during the upstroke", with the amount of lift on the upstroke being "almost none". They concluded that a truck containing a few birds would fluctuate in weight over time, but a larger flock flapping at random would cancel one another and leave the truck's weight unaffected.