alan_m wrote:

If its birds pigeons are the most likely culprit, and if so get rid of
them as fast as possible.

Pigeons here are so fat they'd need a cat-flap to get in.

As I said it was a starling, they do have a history of getting in
underneath the cut tiles around the roof vent. I replaced the lead
weathering slate with an aluminium one, and replaced adjacent roof tiles
when I changed the vent stack from 110mm to 83mm, and put some bird
combs on the eaves for good measure ... persistent buggers.