Chickens Feathers

I have 9 chickens in my back garden they are now approximately 10 months old.  They are very active, noisy, eat plenty, lay eggs every day, have clear eyes, no parasites eg louse or red mite BUT they keep pecking each others and their own backside feathers and legs feathers.  I have dusted them with louse and red mite powder and sprayed them with anti pecking spray. They seem happy run to meet me especially when food in tow, but I am worried about the feather loss.  Is there anything I can do to help them?

If there bum are dirty they could have worms have you checked for that?