You could describe it as biological erosion: a few weeks ago I noticed a small group of house sparrows ‘gritting’ on an old sandstone wall in Horbury.
The sand grains are used in the bird’s gizzard to help grind down the seeds and grain that form its staple diet. Sparrows will also peck at mortar on walls, which gives them an extra mineral, calcium carbonate, in the cement.
Like other aspects of sparrow life – such as feeding, drinking, dust bathing and courtship – this is an opportunity for a bit of a social gathering and the inevitable chirruping dispute.