What are you feeding your chickens. In every flock there may be a few with messy vents. Some of it is because of what they are eating, if they are drinking a large amount of water or wet feed, some may be positional or the shape. Some hens develop a large lower belly for various reasons—fat, ascites, internal laying, etc. If flies are bad in your coop or area, there is always a chance of the horrible flystrike, where maggot larvae are dposited by flies on messy vents or on wounds. So it is good to take a look at them, and occasionally give a butt bath, or just pull off droppings, and trim excess fluff. Sometimes having lice and lice eggs back there will help droppings to gather there. Look for any lice or eggs when you are examining them.
