My parents have about 16 birds and when I visited them recently I noticed that many of them don't look entirely healthy. From under their tail to their partly down their belly are missing feathers and some of them are very red. They also look swollen but I may be wrong since I have never seen the underside of a chicken like this before. I'm concerned that it isn't behavior but we do have six hens who are older than the rest and a different breed, although I have never noticed any rift between the two groups. I only have one picture but I will try to upload more. If anyone has any suggestions or can even point my research in a specific direction that would be helpful. I should add that some of the birds are also missing neck and chest feathers.