She started to puff up like she was cold.
We keep our house cooler (68F) and the outside ambient temperature went below (30F) for the first time so I didn't think much about it. (Not that she would have known what the outside temp was.)
She also lacked some of her customary feistiness.
The following morning she was gone. No other indication of anything wrong.
At least she didn't suffer. Or at least not for very long. It was less than 24 hours from showing something was wrong until she passed.
I can see the other three birds are aware she is gone. The four of them were together since young and they are about 2.5 years old now.