I have ONE chicken among my 5 that consistently gets mites... covered in them, and her feather around her vent look muddy/dirty/nasty. Her vent looks fine, and she lays daily. But usually every 4 - 6 weeks I find her hiding, away from the others, and she has mites everywhere. The strange thing is that the rest of the birds look good. I mean, she is COVERED in mites and I can't even find one on the others. Is this normal?