It was all quiet this morning- I think it must be asleep.
Without demolishing the internal wall there was no way to get to it.
The wall outlet is about 3m away from the extractor fan in the shower, and the bird chirping was 1m below that- so it looks like the vent tube must have become dislodged.
So when I’m home, I’ve got a ball ache job of trying to reattach it and scooping out the bird….