sparrows eating a wall

Came back from the health club feeling all nice and relaxed, but the bloke at the bottom of the close was out with his electric saw, eeeow, eeeow, eeeow.... so I took my coffee etc through to the spare room. Someone was using a motor mower but at least it wasn't as high pitched as the saw. I noticed these birds on the wall of the house opposite. They appear to be eating the wall??!