It's the drakes (of pintail, teal, wigeon, etc) that make the whistle calls.. the hens still quack - tho differently than a mallard. I always have a mixed spread of dekes and try to mix up the calling. I have an old whistle that I removed the ball from that makes a perfect wheezy whistle, like a drake wigeon.
As was said earlier, once the mallards have been spooked a bit, the smarter ones tend to respond to different cues..