Hi.
That looks like rooster damage.
Some mites can be seen, while others are completely microscopic. No, all your birds may not
have signs even if you do have them. Really many parasites and illness will attack the weakest first. Other's with stronger immune systems may not get issues. But yes you need to treat them all if you treat one.
Is she still laying eggs? It looks like she has some pin feathers coming in. I have an EE who lost her feathers in exactly that location after getting into my stag pen and mated by many fighting stags. Needless to say after that incident she quit hiding her nest from me where she came back through that way after laying. However it is taking forever for the feathers to grow back in and she has those same pins on her neck. She is still laying. Feathers are 90% protein. I feed flock raiser with 20% protein. My point is that IF you are feeding layer which is usually 16% it could take even longer for the feathers to grow back in. And I see what looks like carrots and maybe other veggies laying there. So if you feed treats on a regular basis (yes even fruits and veg) then you are likely diminishing the protein even farther. With 16% being a minimum protein level to keep a hen in decent condition, it's important not to give more than 10% of the total daily intake in extras that aren't a part of the formulated ration.
Anyways... it's either a roo, parasites, nutrition, or a bully. Roo being # 1 suspect because of where the damage is. If you still have him you might consider him to be a little too ruff on the ladies. If the others aren't experiencing it, maybe because he has favorites... and they WILL take advantage of the more docile girls. A boy that ruff or persistent would NOT be allowed in my flock, bullies either!
You can supplement her with a little tuna or canned mackerel to help her grow feathers in.... Anyways, what ARE you feeding including treats and supplements?
Ya, my girl is just as UUUgly as yours right now! Lucky her personality makes up for it.
I know you can get this figured out.
