Try giving them spring green lettuce from the store in the winter months. I live in Minnesota. My birds are six months old and have been outside since October when started getting cold. They have done well and some or all of them have been laying 1 egg daily, even sometimes 2. I use an electric heating pad from feed store with water electric heating element. The pad keeps the coup at 20*! I’ll be shutting down the pad as soon as it stays in the teens at night.Mine do this too, but they usually stop after they've laid.
Try giving them some weeds, in order to occupy them.
Mine stop, when I do this.