I noticed on two of my 8 hens that they had lost a patch of feather near the bottom of their backs, before their tail begins. Since they aren't in with the rooster, i knew that wasn't it. I just discovered the problem this morning- those two are roosting too close to the heat lamp! They must not feel any "burn" because they have so many layers of feathers, down, etc. But, time to move the heat lamp up!