I'm not sure about the poopy butt feathers, that's why I got on the site to find advice for my own chickens. I did see some comments on worms and have some experience there. If you start getting poop caked on your eggs its a good sign your hens have worms. I would avoid all the chemical solutions suggested, and just use diotamacios earth sp.??. You can pick up a bag a food DE at your local IFA its only about $14 I think. Mix some in their food. and dust them with it as well as their coop. I suck some up using one of those turkey baster type things that you use on little kids, and blast a little puff on your chickens. Eggs started coming out clean in a few days. Check you girls vent hole and make sure it is unobstructed. Found a broken egg in one of my hens which I just gently pulled out and she was just fine (yes, they do have access to oyster shell).