Poultry lice. Yup. I mix the concentrated permethrin by the gallon. I buy concentrated from the farm store. Read the booklet. Two different concentrations for the pressure sprayer. One for the coop/premise. Second, less concentrated for the birds themselves.
Most easy to dunk them and then let them dry. Second way is to wait until they are good and sleeping and go out and spray each bird one at at time. You have to get down to the skin. Vents, under wings, and then any other place they'll let you. Avoid the eyes.
You must repeat a week later. MUST. If you miss it, you have to start all over again from the start.
Cleaning the coop and treating it as well is also part of the must do's.