You are very fortunate to have wild turkeys at your bird feeders. Your description of flocking, or the joining of families late in the summer, is typical behavior. So, too, is your suspicion that there is no longer enough natural cover for them now that the leaves have fallen. There is nothing you can do or should do to try to help them through the winter. They are very capable of surviving winter weather. Just keep feeding them, and they will be fine.