Page 117 in Chicken Health for Dummies has a table for identifying feather loss and skin problems in adult chickens. Loss on Hen's breast is common in broody hens - I have read that they pull their own out to feather the nest or to give more warmth to eggs. Patchy all over can be lice. Mites can cause loss anywhere - leaves dirty-looking patches of clumped feathers. Head, neck, shoulders likely feather pecking in the flock. You should take a look at the book and consider adding it to your library. Searches in BYC forums, suggested bum feather loss is not uncommon in those who are your best layers and those raised on sand. And the ones with naked butts are the ones that lay almost daily and the ones who don't have more coverage.
I have a small flock of 6 hens, raised together since they were 3 days old. They all got along well, until hormones and egg laying began. Then, I started seeing feather losses, on line searches all led me to look for mites/lice. I never found any with close, multiple inspections, but did treat them anyway. It made no difference. 5 of 6 have some loss on their behinds. But one - lowest in pecking order- had loss on bum and on back. With daily observation, it was obvious that not only was she being pecked by others, but pulling her own back feathers --so neurotic. More reading led me to the answer that their living space was borderline, so I more than doubled it…..Now they don't peck other than a quick spat to get to the best treats, even the neurotic one has feathers growing back.
Make sure you have really examined your two new chickens to exclude mites/lice/skin diseases. If all looks good, then likely they were subjected to overcrowding in the small coop, too much brooding, or an overzealous rooster? Since yours are acting normal, I bet the problem is not serious?
The Dummies book also has a great description of how to do a physical examination on your chickens.
I don't think the brown chicken is Americana - don't think they have that type of comb or wattle?
I have a small flock of 6 hens, raised together since they were 3 days old. They all got along well, until hormones and egg laying began. Then, I started seeing feather losses, on line searches all led me to look for mites/lice. I never found any with close, multiple inspections, but did treat them anyway. It made no difference. 5 of 6 have some loss on their behinds. But one - lowest in pecking order- had loss on bum and on back. With daily observation, it was obvious that not only was she being pecked by others, but pulling her own back feathers --so neurotic. More reading led me to the answer that their living space was borderline, so I more than doubled it…..Now they don't peck other than a quick spat to get to the best treats, even the neurotic one has feathers growing back.
Make sure you have really examined your two new chickens to exclude mites/lice/skin diseases. If all looks good, then likely they were subjected to overcrowding in the small coop, too much brooding, or an overzealous rooster? Since yours are acting normal, I bet the problem is not serious?
The Dummies book also has a great description of how to do a physical examination on your chickens.
I don't think the brown chicken is Americana - don't think they have that type of comb or wattle?
that night hubby puta roof on the temporary coop. My Barred Rock, Cocoa, appeared to have the most feather loss and while this hasn't improved, her comb seems to be less pale than when I got her. The white EE, Alice Blue, had a dirty bottom, feather loss around her neck, and the palest comb in the flock. 
