Can someone tell me what this is and how to treat it? Marek's?? *photos*

[karrm01]

OK, I looked at both orpingtons -- vents look similar. Skin coloring similar. Just harder to find on my unwell one with the disaray and abnormal feather pattern. I didn't see any evidence of mites. Though I understand there are some whitish colored ones. Those might be hard to see on the buff coloring.