I've seen cairns while hiking in mountains and they are usually easy to distinguish from natural rock piles. However, I am a bit confused by what makes a duck and what distinguishes it from a cairn or a natural rock pile. The Wikipedia entry for cairns indicates that ducks are cairns that have "beaks" pointing in the direction of the trail.
On the other hand, this survivallife.com webpage says that ducks are smaller than cairns and can be made from as few as three stones. This would seem to make ducks much more difficult to distinguish from natural rocks piles. So, what is the best way to distinguish between a cairn, a duck, and a natural rock pile?