I have never seen mites or lice on them. Its not running out just messy bottoms.I think it could be mites, but likely just sticky bottoms. My BR and BO's all have poo on the rear feathers all the time, they are super fluffy. Same with the Silkies. Some just always have poo on them. Now if it is running out of them like a slow faucet, then you have an issue(vent gleet)
I am going to stop on the way home for some calf manna & if the sun is still out clean up their bottoms again. It will take me a little longer to get the corn out of their feed since I mixed it all together in a big tub