Nest in roof?

[KE55]

I was rummaging around in the loft today and spotted what looked like a large birds nest protruding through the bottom of the felt in a distant corner.

The odd thing is that the "nest" (if that's what it is) is located in the angle between the roof and chimney, so to build it there from the outside would've necessitated moving the lead flashing and/or roof tiles.

I'll investigate further once the weather improves, but does anyone recognise what it might be?

 

[noseall]

Small birds can access holes not much bigger than a wedding ring.

They are also rather stupid and will keep on stuffing nesting material into holes forever. Looks like a typical native birds nest.