Such a difficult task ruling out what it may or may not be.
As goose pimple says, worm ASAP with flubenvet. It won't do any harm if worms aren't the problem.
Observe them closely for any rasping noise when breathing, any coughing. Are they eating fine. Completely disinfect coop in case of red mite and check birds for mites or lice.
Check eyes and nostrils are clear. The answers to these may help in diagnosing the problem.
Fingers crossed you manage to sort it soon, it is so worrying not knowing what the problem is.
