There are lots of reasons for poopy butts, many are not serious. So you have too look at the whole picture to see what's going on. Hot weather can make them drink a lot which will loosen droppings and may lead to dirty feathers. Some birds just don't push droppings out as far, lazy cloaca it's often called, and that can cause dirty feathers, heavier birds seem to be a bit more prone to that. Internal parasites can cause loose droppings, you might want to get a fecal test done to rule that out. Changes in diet or eating high water content things can do it also, watermelon, cucumbers, etc. If she's acting normal and eating and drinking well, then I'd just keep an eye on her to see if that's all that's going on. If it's really dirty, and you have flies, then a clean up would be good so that you don't get flystrike (flies lay eggs in droppings built up around the vent and the maggots will feed on the chicken - very serious condition). I've had a couple of birds I trimmed feathers around the area to help keep that from happening and keep them a bit cleaner. And if her droppings don't actually look normal, that would mean perhaps more is going on. And get a fecal if you've never done that. Some vets will do it, some will not, you can also use mail in options if needed:
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