We had 10 hens, 9 were a little over a year old, 3 Buff Orpington and 6 Plymouth Barred Rock, and 1 older hen (Dominique) we rescued from a neighbor who left her to fend for herself. One of the Plymouth Barred Rock hens had a bare butt, no sores, and seemed to have no other issues. We found her in the coop the other day, dead in a corner, with no sign of injury from a predator. Recently we have noticed that the older hen (Dominique) is getting a bare butt. She is definitely not at the top of the pecking order and we never notice other birds picking at her feathers. The other 8 hens have no signs of bare butts and everyone seems to act normal. We have been including diatomaceous earth in their Purina layer pellets and have also dusted them with DE. They are supplemented with oyster shell also. They have a fenced yard, covered yard and are locked in a coop every night to roost. Any ideas?