David Sullenberger is a poultry biologist also known as Professor Chicken, http://www.professorchicken.com/, in Santa Fe, New Mexico. He recently gave this response to a question about husbandry for free range chickens in winter:
"Free Range" doesn't mean 'neglected'! Under the very best of range conditions a flock gets only about 28-30% of it's nutrition from the range. The practical average for most flocks is closer to 15-18-20%. Thus, in winter a decent laying flock will increase intake of pellets/grain as the nutrition they can get from the range will drop significantly - say to 8-10%, and in fact it might actually go into negative if the snow is deep (deep for a chicken - lets say over 2 inches) as the birds will expend more energy than they can take in from winter pasture.
Cold nights without supplemental heat will also take their toll - thus high quality grain (corn) along with pellets should definitely be fed in the evening before the birds go to roost.
The photo above is by Traci Torres, http://www.mypetchicken.com/, a site supporting pet chicken owners. This Golden Laced Wyandotte is foraging for food between melting piles of snow.