Many reasons for dirty butt feathers, many of which are not worrisome. Some birds don't push out as hard, so results in dirty feathers. Internal parasites can cause runny droppings which will dirty the feathers. If you have never wormed, or had a fecal done, then I would suggest that, to rule that out as a possible cause. In hot weather they drink more, which can also cause runny droppings, as well as something in the diet that makes droppings runny. If they are eating and drinking, laying and acting normal, then it's probably not serious, but I would rule parasites out if you have not done that. In older hens it can sometimes be a sign of a reproductive problem, which can make it harder for them to push droppings out, normally in those cases they are not laying eggs due to whatever is going on, so if they are laying, then that's probably not the issue.
