A lake I hunt on will from time to time become a roost hole late season when all the food is gone. It seems that most of the duck travel out to smaller tanks and two other large lakes that are shallower overall.
Very few actually go out into fields but feed on smaller stock tanks with 10 miles or so. I've spoke to a couple of the other hunters in the area that hunt some of the smaller water and they typically do much better when we bust our lake up in the morning. It's big enough that it will have roosting ducks and ducks coming into feed at the same time. The ducks that come into feed seem to be roosting on a larger Lake about 10 miles away.
They usually come from that direction and there isn't any other large body of water within 30 miles.