To keep chickens warm, try using a crock pot to supply a small amount of heat.
PHOTO: FOTOLIA/LETICIA WILSON
I have a special setup to keep my young chickens warm. I place a patio block inside the henhouse and top it with a small crock pot from the thrift store. I cover the pot and block with an old washtub.
The young birds perch right on top of the washtub, which is warmed by the crock pot. On warm days, I simply turn off the crock pot.