What symptoms have you seen? How many chickens are showing symptoms, and for how long? Respiratory diseases in chickens cannbe from a virus, bacteria or mycoplasma, and fungus. Sometimes symptoms may be caused by environmental things, such as dust, mold, wet conditions, poor ventilation, and ammonia odor from droppings. Doxycycline is harder to dose for chickens than say, Tylosin, oxytetracycline, or Denagard. If you look up dosages, they are all over the place. No antibiotic will cure MG, but may only help symptoms until the next outbreak. It stays in your flock for the life of the last bird, and it affects most all types of poultry. Close your flock. If you lose a bird, have your state vet do a necropsy where they will test, and identify which of the 8 or more respiratory diseases.