Shoot them on the first pass.... cause thats usually all you will get. If they are sitting and you can hear them, you can usually just whistle back... lots of times they will get up and fly over, or fly over at some point when they are ready to move. They have fairly decent memories from my expereince. Sort of like a turkey remembers where he hears a hen and will often come back and check it out later.

Key is to be ready at first light. Woodies usually fly early... if you have them in your area... just keep at port arms... don't wait on them to work or you won't be eating wood ducks.