How old is the bird, is it acting normally? Laying?
Sometimes poopy butts happen, and it might be nothing to worry about. Could be runny droppings from drinking lots of water if it's hot out. If droppings build up and there are flies around, then flystrike is a consideration, so try to keep them reasonably cleaned up. If it's a constant thing then internal parasites could be a reason. Older birds often just don't push as hard, they can get lazy cloaca's and can get dirty butts. You can also trim the feathers under the vent, if needed, to help keep droppings from sticking there and building up. Dietary changes can also cause it. If the bird is acting fine, laying, everything seems normal it may be nothing to worry about. In some birds, usually over 2 years old, a consistently dirty rear could be a sign of a reproductive problem making it hard for them to push droppings out, usually they will not be laying anymore if that's the case, and the abdomen may feel bloated. She doesn't appear bloated in the pictures, but that is easier to feel than to see because the feathers cover it.
