Eeeek.I've looked at the pics & I really believe its mites. I don't see them on the other birds but I did see them in the nesting boxes yesterday & on the two broodies. They are tiny black specks & Frankie has seen some slightly bigger red specks.
Mites mean you need to OCD clean the coop, and soak all cracks and crevices with a toxin. I would use the high concentration of permethrin in the horse egg fly spray... but it will be a bear.
