I hunt the CA refuge system in the Butte Sink area of N. CA. The area I hunt is next to rice fields. It is true what has been said above about find where they are one day and they will be there the next . I have hunted the same pot hole area that has several rows of trees boardering it and they are in the same spot year after year. It is almost a given that you will bag at least one each trip and often you have the chance to fill your limit (7) if you choose to.
They decoy in with Mallard decoys, rob ducks, Pintail, Teal, or any other decoys. Usually they make up there mind and dive right in. Other times they can be called and once they see your spread they either come in or keep flying. Not much working them like a Sprig or Malards. Action is like a Teal hunt at times.
Usually the only call they respond to is a hawk like sound that I make by drawing air into (not blowing) a 6n1 whistle call. Just one or two calls and not supper loud or they blow right over you. Once you get them to turn , let them find you and stop calling.
At least it works for me
Good luck and hope it helps!!