Cattails and heavy cover. But pass on the sea of cattails. Look for smaller areas of cattails that are in the middle of a CRP field, near a ditch, or just not very wide. Small enough that if you bump the birds they’ve got to fly and not just run to the side aways. Birds in cattails will either hold very very tight and you or the dog almost need to step on them to get them up, or they will circle back behind you and you never know they are there, or they will flush very wild a long ways ahead of you.
Things like being quiet, hunting into the wind, searching for tracks in fresh snow are important. I am young and in-shape and enjoy getting right into the thick of the cattails with the dog, tough work for sure, but it counts as the workout for the day.
Just like with fishing, any little piece of structure can hold a bird. I have found them sitting underneath cedar trees, or in a clump of grass that is still standing but surrounded by grass the snow has knocked down. Tree lines and shelter belts are quite good, but I find them difficult to hunt by myself, usually the bird flys out the other side and I can’t get a shot off.