It's hard to know if those bugs were on the chicken or just outside on the feathers.
I can't zoom in close enough, without focus fuzzing out, to tell what they are.
They kind of look lice shaped, rather than mite-like.
Get a magnifying glass out then hit the internet for examples.
There's a lot of bugs around, very few bother chickens.
Check the birds themselves before 'treating'.
Have you checked them over real well for mites and/or lice?
Google images of lice/mites and their eggs before the inspection so you'll know what you're looking for.
Part the feathers right down to the skin around vent, head/neck and under wings.
Best done well after dark with a strong flashlight/headlight, easier to 'catch' bird and also to check for the mites that live in structure and only come out at night to feed off roosting birds.
Wipe a white paper towel along the underside of roost to look for red smears(smashed well fed mites).