Here are the back/tail feathers of my 49 week old Ancona - she and the 53 week old WL both are looking like these pictures. What could be causing this feather issue and patches where feathers are missing completely? The rest of the flock do not appear to be affected.
What is this and how do I help?
This pic is blurry but shows best the location of the affected area - at the base of the tail feathers on the back

These two pics show best the thin shafts sticking out. Ignore the white flecks of stuff - that's from her dust bathing in the in the shavings.
You can also see in this top pic just before the damaged feathers - the bald spot where her yellow skin shows. Both WL and AC have these areas. I'ved dusted them with Sevin just in case it's a mite issue.


What is this and how do I help?
This pic is blurry but shows best the location of the affected area - at the base of the tail feathers on the back
These two pics show best the thin shafts sticking out. Ignore the white flecks of stuff - that's from her dust bathing in the in the shavings.
You can also see in this top pic just before the damaged feathers - the bald spot where her yellow skin shows. Both WL and AC have these areas. I'ved dusted them with Sevin just in case it's a mite issue.