I agree with @ChickNanny13 that this is dry fowl pox virus. It usually lasts about a month and just make sure they are eating and drinking. The wet version of pox causes yellow lesions inside the beak and throat. If any seem to be especially sick, look for those yellow plaques. If any scabs form on or around the eyes, you can use triple antibiotic ointment on those. Don’t disturb scabs though, since that can spread pox. There is a fowl pox vaccine available for those who have not been exposed. The ones that recover will become immune to fowl pox in the future.