I've seen many people say, quite categorically, NEVER use straw as mites love it. On the other hand I've seen an equal number say, equally categorically, NEVER use hay (but have never given a reason). So, apart from offering them pistols at dawn, who do I believe?
Hay gets caught in their feathers, if they eat it, the long pieces can block the crop, and could lead to sour crop.
Straw, because it is hollow, it can be quite snuggley warm when fresh, but its not so good when wet, needs to be turned over/cleaned frequently. It is no greater habourer of mites than the wood of the coop itself.
woodshavings..MUST BE PRE DUSTED, ie dust removed, as the dust can cause respiratory problems, likewise sawdust. Shavings must be good quality, as sold for horse/rabbit/poultry bedding. DIY type woodshavings may contain hardwood(not good) green wood(not good) or even traces of plants/fungi unknown. It is reasonably chip, VERY easy to pick,compost, absorbent.
hemcore- I think it is chopped hemp straw, Is a personal favourite as it is super absorbent and smells nice. costs more than straw, but slightly less than shavings. it works well half and half with shavings too.
straw is preferred by the hens for nesting, as they pick it up in their beaks and move it around(usually ineffectively)to make a nest to lay in...if you dont have a seperate nest box, if you put straw in one corner of coop, shavings/hemcore everywhere else, they choose the straw corner to lay.
does that help?