Most chickens have some mites. Mostly just a couple and it's not a problem and they can control it with their dust baths. But it can get out of hand and become a problem and sometimes these little things are almost impossible to see. That's why most of us dust for mites as a precaution, to stop the problem before it starts. What you can do to help keep the mites under control is fill their dust baths with wood ashes. Ash helps keep the mites under control and it's harmless to the chickens and it's also a more "organic" method. If you do have a serious infestation the best route would be a chemical poultry dust though. Also treat their coop and run for mites occasionally, as a precaution. There are some products you can spray the coop with. Ask at your local feedstore what they'd recommend.
