We have a flock of 6 hens and a rooster, 2 light brahma's, 1 hen, 1 rooster. The hen has a nasty looking butt. When I went out to take a picture, she pooped, so pic of it too. None of the others have this issue.
Believe me that is not that nasty, LOL. I have a couple that look much worse, and require some extra care. Some chickens can have a bit of diarrhea or some extra fluff around their vents, which can make the vent area look a bit dirty. You can break of any solid material when it has dried, trim a few extra feathers, or during warmer weather (or inside the house,) give their bottoms a quick wash. It's always a good idea to look more closely at the vent area when you have cleaned them up, for signs of a prolapse, redness, or for the pressence mites or lice.