Mites and lice are more common than fleas on chickens...someone will chime in soon with how to treat them....but if you want a faster answer use the "google custom search " on the top of the page and seach for Mites/lice treatments....there are several threads going on all the time about this common problem
Not sure if chickens get fleas, I suppose it's possible. The scratching and biting may just be an itch or preening - but could be mites or lice too.
I would start with either sevin or poultry dust. if you suspect either - remove all the bedding, dust the birds and the coop, place new bedding in. REPEAT in 7-10 days to get any nits that hatch. Lice aren't too bad - mites are a real pain to get rid of.
You may need to go to Ivomec for mites if the dust doesn't work. Check your birds to see if you can see anything. Lice look like small tan rice. Mites are really fast and hard to see. The signs of them are feathers around their vent look like they have dirt in them. Could be other places too, but generally on the vent area.