depends on what duck you are hunting. there are calls for mallards, pintails, scaup, woodducks, teal, etc. Most of the guys I hunt with in the midwest have half a dozen around their necks. Foiles makes a variety of really good calls.
Guys do that down here, whistles for teal, widgeon, and pintail, mallard call, etc. We don't have that many mallard in the marsh, only a few in one fresh water hole down there where I hunt, mostly salt marsh stuff like Widgeon, gadwall, teal, scaup, smilin' mallards (shovelers), other such. Mallard call with call ALL these ducks except one, the squealers, Mexican tree ducks. They are sometimes still here opening week and any amount of calling with a mallard call is futile with 'em. They don't much like decoys, either.
Anyway, I just go afield with a mallard call and a snow goose call. All geese down here, lesser canada, specks (white fronted), rosses, and snows will come to a snow goose call. We have danged few greater canadas, but I have called a few with a snow goose call. Snows are the predominant goose down here.
I used to have a whistle, never really did much with it. I'm not sure where it went, lost it in a move, probably.
And, ditto what RC says above on steel shot and chokes. ^^^^^^^