So it's shavings rather than wood chips. It sounds like dust bathing then. Ideally they like to dust bath in dry earth. They will scratch up the soil in an appropriate area creating a dimple in the ground and flick the dry soil over themselves, wriggling and fluffing their feathers as they do so. The fine soil helps to dislodge any lice in their plumage. If they are confined to a run and there is no suitable place for them to do this (perhaps it's too wet or a concrete base), you can create a dust bath area for them using an old baby bath or something of a similar size filled with a mixture of sand and soil and wood ash or Diatomaceous earth (DE)
To check for mites and lice, lift them off the roost at night and turn them on their backs and utilising a head torch, check their skin around their vent and under their wings(armpits) and back of their necks.
Mites are usually red because they suck the hens blood. They live in crevices in the coop during the day and crawl onto the chickens at night to feast on them whilst they are sleeping. When you are cleaning out the coup during the day, use a soft brush to sweep into the joint where the roost meets the wall of the coop.... if you have removable roosts, lift them out and sweep the socket and the underside of the roost bar into a dust pan. Tip it onto a piece of white paper or use a white/light coloured dust pan and you should easily see red mites if you have an infestation problem. If you are unsure, crush one with the back of your finger nail and it will stain the paper with blood. Obviously, if there are a lot of these your hens can become anaemic due to loss of blood and this will affect their egg laying ability as well as general health and make them susceptible to other ailments..
Lice live on the hens all the time. They are usually small and grey/brown coloured. They lay their eggs at the base of the feather shafts, particularly around the vent area. If your hens have a problem with an infestation of lice, you can usually tell by the clusters of eggs attached to the base of the feathers.
If you do a search for images of poultry lice and mites before you do it, you will have an idea of what you are looking for..