Bald patches on a few of my birds... any idea for causes?

I know in the first two years that we began keeping chickens again, we had some problems with picking... one of the birds got some ruptured feather on her back, a little blood, a little picking, more blood, more picking.... you get the idea. We basted her bald back with pine tar and this eventually "cured" her of her pecking appeal. (except the time when I neglected to re-smear the pine tar on her and came out to the coop to find a large hole pecked in her side... that chicken was so good while I cleaned the wound and blew dry her damp feathers). Anyway, since this, it kind of calmed down... except for the great feather fest when we got some straight run banties (4/5 were roos) and some "sexed" buff orpington hens (2/5 were roos), and had 6 roos against 14 hens, we haven't had much trouble.

The coop has calmed down, we have one big friendly rooster and 10 hens, but the problem is the rooster has never had a full tail... His butt above his vent is red, spotty and plucked all the time. He also started developing a slight "hole" in the feathers on his neck which has now grown into him having a patchy neck, lots of feathers trying to regrow, but none actually doing okay. (I think part of the problem here is, I noticed one or two of the feathers are kind of corkscrew looking... spiral like, and it attracted too much attention...). I honestly think the hens pick at him! He is such a gentleman and is very nice to all the hens.

Then most of the hens are looking great, no bald patches, nice tails, no poo around the vents... but two or three of them have bald butts above the vent and/or poo clinging there below/around it. One also has a bald throat. It frusturates me to see them looking so shabby. We did have mites attack and kill a hen six months ago, but I think it was because she was old and weak... we pressure washed out the coop and dusted all the birds, but never found a single mite on any bird but her.... it was very strange.

Any wonder why this is happening? Poor Big Red (the roo) is looking shabbier and shabbier, although he isn't acting any different than normal.

(these are all buff orpingtons by the way... all probably 2-5 years old)

Thanks!


EDITED TO ADD: Oh, I also forgot to say that I don't think they are overcrowded at all... they have a very large shed with tons of roosting and nesting space (8x8) and a very large outdoor run (don't know the exact dimensions but it it's over twice the size of their shed) with outdoor roosting options too.

 

Ugh, I don't know. We dusted for mites a few times, no matter which bird I check I still haven't found any.

I do agree someoen is probably a feather picker, but I can't see who! They all look the same (except for the baldy-waldies of course!).