Thankfully I had some leftover Enrotril syrup that was only a month old, prescribed for a bird that died of a staph infection (as far as I have been able to find out, veterinary antibiotics are only available by prescription here in Australia).
The pullet was separated from the flock immediately and given 0.3 mL Enrotril syrup, twice daily then daily.
The next morning the breathing was normal. She was very active, flapping vigorously and complaining loudly when handled, with a good appetite.
I think I caught it early enough, and have released her back into the flock, after 3 days in the box. She seems fine today. No other chickens are affected.
She’s no beauty but I hope she’s ok!