thanks for the replies & sharing those articles. Helpful information.
Interesting mention of switching feed, as we have, our local agway closed up last fall and now get feed from either Runnings or Tractor Supply.
Have always add a tablespoon apple cider vinegar to the water, but maybe not...
6 hens in coop and all have dirty vents to some degree and a couple have constant dirty/seeping vents as shown. They are healthy otherwise, eating & drinking etc. Seems to have come about over the last few months. Have raised chickens all my life and don't recall ever having a hen look like...