Pullet with rattled breathing
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Have you recently gotten her, or have you recently added another bird to our flock? This sounds like either infectious bronchitis or mycoplasma (MG) but there are other respiratory diseases in chickens. I would get some Tylan 50 injectable with a few syringes and needles at your local feed store, cattle section. Give her 1/4 ml orally twice a day for 5 days. If you can't find that get some oxytetracycline or Tylan powder, and treat her in her water. She should be kept warm and separate from healthy chickens. Here is a good link to read about all of the common diseases including the respiratory diseases:
One last question...how long should we keep her isolated? I!ve read anything from 5 days to 2 weeks.