Can you look under the vent and lower belly for those bugs, and for any white clumps of eggs at the base of feathers? I don’t know if those are mites or lice, but permethrin dust or spray will treat them. Depending on which they are, you need to treat them at 7 days intervals for mites, and at 10 day intervals for lice. Treat at least 2-3 times to get the bugs, and then the newly hatched ones before they reproduce. The coop, nest boxes, and roosts, needs treating after bedding is emptied and deposed. Here is a good link with pictures of lice and mites, and certain mites only come out at night:
https://the-chicken-chick.com/poultry-lice-and-mites-identification/