Here is a little tip I picked up from an ancient hen keeper, it might be of some use to someone. Can anyone remember the old salt glazed stoneware hot water bottles that people warmed the bed with at night before rubber hot water bottles? Not used for many years I know, but some people kept them as doorstops etc. They make excellent sources of heat, when filled with hot water and covered with straw, in the hen house at night. Tried this out and it keeps the hens cozy for the price of a kettle of hot water.