Ambush! If possible, as has already been said, find out where they roost and how they get to the field. If you can get in between, that would be best. If they fly from the roost directly into the field and back to the roost, I would try maybe ( while they are in the field ) , a Redtail Hawk call, Great Horned Owl, Coyote howl or something predatory like that. Maybe scare them out of the field. Turkey's pretty much do what they want, when they want so, if these things don't work, you may just be stuck watching them. Good luck!