Ducks are objects that appear during gameplay in all of the RollerCoaster Tycoon Games. They are based off real world Mallards. Having water in your park allows ducks to fly in from the side of the screen and land in the water. Ducks swim in a random pattern and may sometimes end up on a ride at the surface of the water. Clicking on a duck triggers a quacking sound.
Ducks fly off after spending some time in the water. Removing the water from underneath the ducks will cause them to immediately fly away.