Most of the fights are with roosters that are less than a year old. Some of the hens have been fighting too. But they fight more with the red heat lamp on.
My chickens are slightly overcrowded, at 93 in 140 sq' ft coop. Many small breeds (silkies, nankin bantams) and many are young (6 months or younger). Plenty of roosting space. They free range in the day and fighting has only been a problem this week, as soon as I turned on the heat lamp. When I turn it off, they calm down quickly, turn it on, and they're nasty. So where the coop is smaller than the number should be (I think around 75 is correct), they rarely squabbled until the heat lamp came on.
Odd, no? Whoops - I guess I should have posted this under behavior, but I'm also nursing one of the roosters.