I am unfamiliar with her breed, but she is a cutie.
She may be molting and her feathers that are left look ratty maybe because of their age... But I WOULD definitely treat for parasites... the skin is red, indicating irritation beyond just molting. How are your current temps?
It true that feathers are made of 90% protein and it's amino acids. And that it takes lots of energy to produce eggs and feathers... it's why they don't do both at the same time.
You might consider switching all you birds to a grower or flock raiser and offer oyster shell on the side... no need to worry about it getting eaten up from her. This is what I do for my birds, .. since those who are in molt don't need the extra calcium and fed long term to birds not in lay *could* (doesn't mean will) cause gout and even kidney failure. Actually excess protein could cause the same thing but a different type. But that's most likely in birds who are predisposed genetically somehow and usually also over feed treats. All your birds will benefit from a little more protein. 16% is the MINIMUM for light bodied layers like leghorn. But 22% was shown to give the highest hatch rates... meaning more nutrients going into the eggs our family is consuming. At one store the flock raiser (20% protein) is equal price to layer, since oyster shell is cheaper than protein, it's worth my investment even if it were slightly higher price. Sometimes I MIGHT mix gamebird or turkey starter with my feed to get a higher protein content. Even then, not all are created equal, read the protein and calcium levels to know what you are truly looking at. I stay at 25% or less long term. It DOES heavily impact how fast feathers grow back in and the birds return to lay. However, I keep nutrition up ALL the time so that they aren't starting out already depleted. And even though I do this... genetics is still a factor to how fast they will molt. If you wan't just this gal to get extras, separate her out for a few moments.
Would the bugs only be on her and not the others?
Studies show that in back yard flocks 90% of parasites were on 10% of the birds. Thing is they all have different immune systems and foraging/dirt bathing habits. So yes just like human families, things can effect one member more than others. If she truly were a runt and it isn't just her breed/genetics making her smaller then that would be a good indication that her system is not up to par with the others.
That being said... just because others aren't showing symptoms doesn't mean they aren't fighting it. If you want post a pic of them so we can look at feather condition. However, even if nothing is identified if you treat one, all should be treated. Thing about parasites... Some of the mites are microscopic (depluming and scaly leg mites) and will never be seen with the naked eye. Some others don't live on the birds and only come out to feed on them.
I would need to know if she were laying if she were mine. I'd be watching. I usually recommend permethrin based product to fight off most parasites as it is affordable, easy to use, and effective on most pests, with no egg withdrawal required. BUT I might consider hitting that gal with Ivermectin pour on as it will treat many internal and external parasites.
Lot's of good threads on here.. how to transform a muddy run. Without much dirt bathing opportunity parasites can get worse... if you don't have something already... consider using an old shallow rubbermaid tub, old tire, or whatever and make an elevated dirt bath somewhere under cover so it can stay dry.
Meal worms and scrambled eggs are good supplements or treat... but both are VERY high in fat and would NOT be something I do daily.
@azygous I know you have experience with gals and weird molting habits... have ya got any insight or suggestions here? Thank you.
