"Could someone please explain to me what a ducked route is? And, also what is a duck trail marker?"
Last things first: A duck is a minimum of 2, but usually 3, stones stacked one on top of the other, and is used to mark an indistinct trail or off trail route. A ducked route is one of the above mentioned routes with ducks strung out along the route to mark it. Another type of marker is a cairn, which is a pyramidal stack of rocks used for the same purpose, where the base of the pyramid consists of more than one rock, as opposed to a duck where only one rock is in contact with the ground. At least that's the terminology I learned in California. Up here in the soggy Northwest, they are all called cairns. Terminology varies from place to place, but the idea is the same.