Hi, welcome to BYC!
Depending on how you check for lice and mites can be very difficult... and even impossible to see. Some mite are microscopic, so you will NEVER see them. And on top of that, the red mites do NOT live on the bird. They live in cracks of wood and come out at night to feed on the birds.
Best time to check is a couple hours AFTER dark. Keep the light low and using a flashlight hold each bird on their back, legs toward chest, part feathers near vent and on abdomen and look for things running away. You can also run a white paper towel under your roost and check for red smears (same time).
I agree that your girl has an issue. I don't think it's nutritional. Just be sure not to diminish protein ever below 16%. I have a girl that looks the same. Took to the vet in case I was missing something. He said very likely there were lice or something previously. I treat with permethrin spray. It is very effective and safe as well as affordable. There are organic versions for more $ if that's important to you.
Another thing about bird lice... there are 3 types, I believe. Head, body, and feather. The head lice will not travel to other portions so you wouldn't see them near the vent. Also, this is your only girl SHOWING symptoms. Doesn't mean your other aren't affected. They may have stronger immunity, fighting it off better. So be sure and treat ALL of your birds. Very often it will be the same birds that have the most issues. They are more susceptible than other for whatever reason. It is said that 90% of parasites were found on 10% of the birds in studies.
Please note that even if you get this figured out and treated... her feathers will not come back to normal until she molts.
From what I can see, she looks like a beautiful bird!
Personally, though I'm against treating for what you don't have... I would treat right away, even if I didn't think I saw anything... because to me those feathers are very indicative.
One final note.. depluming mites (and scaly leg mites) are invisible and live inside the feather shaft, cannot be treated with non systemic solutions. But you should see skin irritation, maybe lifting around the feather follicle.
Best wishes!
