I can easily smell the shrimp when they are eating them. Many are sometimes found live in the feathers, and they come in on the decoy anchors that snag underwater plants. Never any shrimp taste to the skin or flesh after baking out the fat. I'd guess the ducks are grubbing the bottom for invertebrates. You should shoot a few and check. Like scaup, canvasbacks, and redheads, they just don't have the bill structure for grasping fish.
I grabbed my double 12 and scouted for four hours in the rain this PM. Only the wheat has been harvested; the remaining 90% is corn and beans, with hundreds of acres in snow and in standing water. Many roads closed. Water everywhere and rising. Saw very few ducks, but that is understandable as the bird stay undisturbed in flooded beans all day. Felt sorry for my GWP Gus,who wants to hunt so bad!
