Don't stress about catching her yet.
Is your feed a pellet or a crumble?
If not I HIGHLY recommend getting a pelleted or crumble feed. The number one thing in your control to prevent illness is what you feed. It may be too late for some birds but for the future a balanced diet is worth its...
Welcome. Poor girl might have something serious going on that you may not be able to fix. Is her belly full and squishy feeling?
Is a vet an option?
I know you mentioned you aren't sure of their age... Which probably means you have no clue how they've been fed. What are you feeding?