It appears from the photo there are no abrasions or open wounds. Correct? If there are minor abrasions, spray with Vetericyn.
The baldness is more likely a panic injury than mites. This happens when a chicken has never seen or heard a shop vac or seen a big dog or a bear, you get the idea. They panic and try to squeeze to safety through impossible openings, sometimes even flying straight into a wall. This often causes such injuries, mostly minor in scope.
Try to think how they may have been frightened into panic, where it may have occurred, and what might have caused such an injury and you might want to fix it.
