Here's my take on pasty butt. I think it happens when the chick touches the shavings/ground with its butt while pooping. So some of it sticks to the butt. Then the next time it touches it again and then it has a build up of poop stuck and eventually blocks up its bottom. I believe that is the problem because people have a hundred reasons that they think causes it but it still happens in every situation. That us also why I think some adult chickens seem to have funky butts. If you watch than you will see that they squat too close or even touch the ground while pooping do some gets on those feathers. . As the feathers get more build up of poop it becomes a huge mess. Most chickens have feathered butts but most don't get poop build up cause they don't squat so low. Give me a chicken that shoots poop out without squatting!!!!
Well now I never noticed, but this one chick I've got was on paper towels for the first few days and still is not that I think of it. I just got new shavings and haven't put them on any. There's only the two.
Pasty butt is smaller and skinnier. I've cleaned her

fl ) butt a couple of times. Too she was put in with the Sussex hen who didn't bother with her really but then Pasty didn't keep up with the others either.
I normally keep brooded chicks on medicated feed but not these.
You have a point since I clean her but the poop isn't necessarily stuck at her vent. Still she doesn't seem to be thriving like her cell mate. Both are Marans.
Speaking of which how are yours? I've been in contact with some one selling Birchen eggs but can't afford them right now.