We used to HAMMER them at our place in SW Louisiana. We call them Squealers. Our marsh was full of them for about a decade. Large flocks of the big, dumb, slow flying ducks were easy to shoot and could make for quick limits. After a while, my cousins that live down there let most of them pass for the big greenheads, Pintail and other puddlers. Not much of a challenge. They would come to a whistle, but calling wasn't really necessary. They would light on mud flats, sand bars and trees near a food source or loafing spot. Never saw one land in water though.
The are , by far, the best eating duck in the marsh. Much better than teal or wood duck, in my opinion. Surprisingly, they're not near as gamey or ducky tasting as the puddlers we kill. I didn't believe I was eating duck the first time I had them. Great table fare plucked or just breasted out. Cook them to medium rare and you'll be amazed.
We still kill em out there pretty regularly, but there's not near the numbers we had in recent years. I hear that some have migrated as far north as Michigan and are showing up on the Atlantic flyway as far as North Carolina. That's kind of exciting.