Why is a large flock of Sparrow’s descending on the crazy paved patio and pecking/eating the newly pointed joints between the paved slabs?

This is a real mystery. A friend of mine has sent me a video of some strange activity, with a large flock of sparrows descending on his recently re-pointed joints of a paved patio. He’s used a concrete type mix (postcrete), that is quite coase and has some very small stones etc in. The pointing is fully set and hard. It’s really odd behaviour and would love to understand why they are doing it.