Tail feathers on my chicken damaged? What is causing it?

[Jakkals]

I have 5 chickens that are one year old and 3 weeks ago my Barred Rock stared displaying some damage to her tail feathers. The other 4 do not have the same issue. I looked for lice but nothing visible with the naked eye. I dusted her with Diatomaceous Earth twice but no positive results. I have included some pictures. I tried looking online but did not see anything helpful.









Thanks for any help.

 

[Eggcessive]

Your other hens may be picking at her feathers, or if there is a rooster, she may be getting overmated. Lice/mite eggs can be seen next to the skin especially near the vent and under it. DE does not work if you have a major infestation--use permethrin or 5% sevin dust. Here are some links with pictures to read:
http://www.the-chicken-chick.com/2012/08/poultry-lice-and-mites-identification.html
http://anrcatalog.ucdavis.edu/pdf/8162.pdf

 

[Jakkals]

Thank you for the feedback. I will pay attention to your suggestions and see what works.