keeping hens laying in the winter

We cover our Suscovich-style  chicken  tractors with  clear  plastic to  make  mobile  mini-greenhouses  for  the   laying  hens.  (This picture actually shows the tractor with its summer tarp cover, before we switched out for plastic.)  Right  now,  the  day after  Christmas,  we're  getting  about  nine  eggs  a day  from thirty-five  hens,  mostly  two-  and  three-year-olds.  We  think  this  is  partly  because  of  the  warm  greenhouse  atmosphere  inside  the  tractors,  which  melts  the  snow  and  encourages  the  winter  cover  of  wheat.  Generally  it  is  the   birds  in   the  tractors  that  lay  best,  better than the  coop  birds,  who  have  a warm  house  and  access  to  a large  fenced  paddock  in  the  garden.