I think part of it is space and the fact that mama encourages them to move around.
Also the constant heat. I noticed chicks under a bulb don't run around as much as chicks that have a brooder plate for warmth. Brooder plates are safer and use way less electricity.
I haven't actually noticed much difference in feather growth between winter brooded chicks and summer brooded. The hens do a good job keeping the chicks warm in winter and I always have them with the flock during that time for extra night time warmth.
Also the constant heat. I noticed chicks under a bulb don't run around as much as chicks that have a brooder plate for warmth. Brooder plates are safer and use way less electricity.
I haven't actually noticed much difference in feather growth between winter brooded chicks and summer brooded. The hens do a good job keeping the chicks warm in winter and I always have them with the flock during that time for extra night time warmth.