Those look like ticks to me. Can you try to pull one off with tweezers? Then clean the skin with iodine, alcohol, or Vetericyn.
Ticks can cause a variety of illnesses in chickens including spirochetosis, a paralyzing disease, anemia, and weakness. Treatment recommended by the Merck Veterinary Manual includes pressure washing the coop and treating it with permethrin spray, or other insecticides, and I would dust or spray the chickens as well.