I agree with Purpletree, you need to treat immediately. While your at it, look at your other birds and see if they have any. Chances are, more than one bird will have mites or lice if she has them.
Make sure to remove all of the bedding from your chickens coop, and spray/dust it well. Sevin dust works great as a mite killer, but there are other products like Poultry Protector that can work as well. You'll likely see the lice reappear eventually, as mites and lice are found in the envirorment. Even if you get rid of them on the birds and in their coop, they can still come back if a wild bird introduces them, or if they just happen to get there from surrounding areas.
Remember to treat her again in 10 days, as any lice eggs that you missed will hatch by then.
Don't be too hard on yourself for not noticing the lice before, as its pretty easy to overlook them if you don't expect to see them. I just noticed that one of my roosters had them. I treated him for lice not too long ago, and have been checking him every week or so near his vent and under his wings (the normal places for mites and lice). However, I looked at him just yesterday, and was surprised to see mites in other areas of his body, not where I'd been looking. Instead of having mites in the normal places like the vent and beneath the wings, he has them in the feathers on the surface of the wing and beneath the head feathers. I don't know why I didn't notice them before! Fortunately, I've treated him again, and he's now mite free (I hope!)
Especially on her neck and they're all crawling around her ears and face.
