What do you have available to purchase?
Ivermectin would also work but you really need to treat the building if it is infested. You should probably go out after dark with a flash light and really investigate and see what it is you are dealing with.
Do the birds free range and have a lot of dust...
It sounds like it could be a louse you saw.
I would suggest you buy some 10% permethrin and dilute it per the label to treat first the coop then the birds.
You will need to completely clean out their coop. I would also vacuum the crevices with a shop vac then spray all surfaces of the coop with...