I had a college fried, Freddy, whose father owned all of the land surrounding a large slew right in the middle of the North Dakota central flyway. He left a bordering field of millet (bird seed) unpicked to hold the ducks and geese for a large part of the winter. They would raft up at night and keep the water open. In the morning, when they rose, the first wave would beat the second wave back down to the water. We sometimes couldn't see them but we could hear the commotion.
We shot as many as we wanted but were not allowed to waste a single bird. It got so that we would just shoot the green heads out of a flock. We would clean every one and Freddy's mother would cook some for us and can the remainder. She knew how to roast wild Mallards. They were delicious. Freddy and I would stop at a grocery store and load up on things that made up the duck dinners. We were happy and Freddy's parents were happy. Alas, the passing years have ended that for us.