Every year, thousands of guillemots migrate to Stack Rocks, just off the coast of Pembrokeshire. Their eggs are slightly pyramid-shaped to stop them rolling off the steep cliff-edges. Every egg has a different Jackson-Pollock-like set of scribbles on it, so that their parents can identify it.
Harry Mount visits Skomer, Pembrokeshire, for the 400th issue of the Oldie, out this week.
