Caught in the Act

It was a puzzle how so many clumps of lichen were landing on our patio, but the culprit was revealed when I zoomed in on this pigeon on the roof.  No idea why they tear it off and throw it down, as he didn't seem to be eating anything (insects?)

This was a brand new roof about eight years ago, but looks like it's been there for decades!  No pollution here (in spite of all the diesel cars!)