She looks to have been cut by a roosters spur, or suffered some sort of injury. Can you bring her inside in a crate for warmth? I would soak her in some warm soapy water, weak betadine, (or Epsom salts) to clean off some of the dried scab, or spray her wound with Vetericyn. Apply some plain neosporin or bacitracin twice a day to her wound. Dry her thoroughly. Her wound should heal fine in a couple of weeks as long as the others don't peck her. When it starts healing BluKote may be good to hide the wound. Place her crate back outside with the others in a few days to stay part of the flock. Watch for signs of infection, such as pus, redness, or swelling. Blue green bruising may be noticed on the skin.