Testing is the best way. Usually MG causes eye bubbles, eyelid swelling, and they may be a bit lethargic. IB is more mild and symptoms are usually sneezing every few minutes and a bit of congestion. IB also spreads through the flock over weeks to months, where MG may only affect one or a few.
I would suspect that one of the new chickens is a carrier for infectious bronchitis. Tylosin and Tylan are the same antibiotic. It can treat MG, but not IB. Tylan 50 injectable is sometimes found in feed stores along with syringes and needles. It is usually given orally in chickens. Dosage is...
It sounds like she is congested and sneezing. She may have early signs of a respiratory disease, such as infectious bronchitis or a mild strain of mycoplasma gallisepticum (MG.) How long has this been going on, and are any others doing it? Have you added a new bird to the flock recently? I...