Is there a bad odor coming from them when you get close? It appears to be one of the many avian viruses (Infectious coryza).that can afflict chickens. I hope you have no chickens of your own because these viruses are wickedly contagious.
Many of the ailments stemming from respiratory viruses may respond to Tylan, but that will help the secondary infection while the virus will live on in the chicken's cells for the life of the chicken and even beyond in the environment.
A couple years ago I was a few feet away when a hawk dive-bombed a young six-month old pullet. The feathers flew in every direction like an explosion in a pillow factory, but when I finally found my pullet, she had a perfectly bald back but not a scratch on her skin. She was lucky, probably because she was fast.
I would bet the hen you are wondering about had a similar close call if she was out free ranging or if your run is uncovered.