I have a flock of 6 hens (all kinds) and one roo. They're very healthy, active, and laying 5-6 eggs a day. My BO hen, Sarah, has had a dirty behind for about a week or so. She is VERY plump, and at first I thought that could be the problem, but this morning I noticed dark brown, runny poop all around her vent. She's eating normal (she's the greediest of all my chickens), and seems to be laying. No "sick" behaviour, other than the runny poop on her behind. I give all of them jogurt every day, and their water has ACV in it. What do to?