Pekin duck with dirty tail feathers? How to treat?

[Stellajj054]

Hi, I noticed one of my pekin ducks had dirty and missing tail feathers and was hoping someone could help figure out whats wrong and how to treat this. Noticed it a couple weeks ago and got em to check and maybe clean around but couldn't find anything. It did get better but today I woke up to check and it’s definitely gotten worse. Any help would be greatly appreciated!

 

[nuthatched]

Do they have a pool? Any drakes?

 

[Stellajj054]

Yes, they have a pool we clean out almost every day and no drakes