Your birds could be having a respiratory infection. A broad-spectrum antibiotic like oxytetracycline can be used.
Also, important to isolate the sick bird/s and offer treatment to all birds because most respiratory infections are transmitted through the air.
Avoid mixing birds of different ages and always isolate new birds for 2-3 weeks before mixing them with the current flock.
Hygiene is paramount as well as cleaning and disinfecting your feeders and drinkers always to keep disease pressure at its lowest.