Those are lice eggs. My chickens had lice last fall and the eggs looked just like that. Give your chickens a dust bath to help get rid of the lice. Also, clean your coop to get rid of any lice that fell off the chickens.
Once I got five Cornish x Rocks from Tractor Supply. At least they were supposed to be Cornish x Rocks. I noticed one of them was a lot smaller than the others. We also got 20 other meat birds of a different breed. I forget the name of the breed. As they grew, I started calling the small one...
I don't know what caused it, and I don't know a treatment, but I would separate her from the others until she gets better. Give her her own little cage to live in, or if you have a extra coop, use that. Make sure to keep her warm, and give her lots of water. I hope this helps and that your hen...
I have a Easter Egger named Hawk. Her other name is Goose because when she clucks she sounds just like a goose. She is the second chicken in the picture.