1. Keep warm. Just don't let the cold wind blow to budgerigar, because whether it is diarrhea or a cold, you should pay attention to keep warm.
2. Stop eating vegetables, fruits and greasy food.
3. Observe whether there is diarrhea or runny nose in the stool, and consider giving some anti-inflammatory drugs appropriate intervention.
The above is only personal farming experience, for reference only. No wonder bird friends make mistakes in the discussion. I hope it helps you.