How to get the ball of stuck poop off my hen's fluffy butt

[PetRock]

I didn't have to do it when they were babies or even the first few months of laying. But I have a few of my 1 1/2 yr olds who I guess aren't that good at the art of elimination. The worst offenders are my two Barred Rocks and my two Silver Laced Wyandottes. At least they don't mind being bathed every so often.

 

[DebbieDee]

I guess I'm lucky too. I've never had to do this. It's a bit cool in these parts now. I hope they don't get chilled.What causes this?