@oesdog when it's my turn to go, I wouldn't mind going like your Isobella...be active and fine one day and gone the next. She didn't suffer and lived a long, comfortable life. It stinks to miss them but that's where it's nice to have happy memories of them.
Chicken math isn't always a bad...
I think Aggy has loved her life with you & your family so much, she hasn't been quite ready to go. If she isn't in pain, I'd just let nature take it's course. It doesn't sound like it's going to be a lot longer and you don't need to put yourself through having to do that.
I think you are right that they know. They're probably better at that than people are. It sounds like she has been a wonderful bird for you. I'm glad you have had her and enjoyed her. It's hard letting them go but good to know she isn't suffering.
OES, I'm sorry you're losing your beloved Aggy. It's never easy to lose the animals we get attached to. I'm sure you've given her a good home and she's had a good life. Sorry for your loss