I just yell "Hey Ducks!"
I guess I'm not as creative as I'd like to think I am.
When we first got ducks, I thought about whistle training them, but I can't whistle so it would have needed to be a metal or plastic whistle. Then the idea of having to remember to have the whistle on me every time I need to call the ducks seemed terrible. I had a little wooden train whistle too, but then realized that with my luck, as soon as I trained the ducks to come to the choo choo train whistle, the train by our house would come through for the first time in 10 years and my ducks would run off like a bunch of hobos with the train. (totally wouldn't happen, but I live the image of my ducks on the tracks with little bindles.)