Chicken tail feathers look chewed

by jeanette
(England)

My 3 year old ex-batt hen is healthy although
perhaps a little overweight. I have noticed
the end (not where it joins the body) of her tail feathers look scruffy and some just have the central
shaft. Some of her back feathers look not smooth.
She doesn't appear to be scratching and checked for
lice eggs on shaft and can't see anything. Should
I treat with frontline as a percaution in case it is
northern fowl mite?