chicken with red sore-looking rump

[Russ]

I'm new to raising chickens. So, I'm not sure what to make of the red rump that a couple of them have. There is a round patch on their "butt" area that is missing feathers and is a deep red. It looks like it would be sore.

Any ideas?

 

[s6bee]

How old are they? This could be feather picking due to stress, check them for lice too. Pictures could help but. How much space do they have in the run and do they get any free range time?