Common guillemots breed on narrow open ledges, packed side by side, and build nothing. A single egg is laid directly onto rock, where a nudge from a neighbour could send it over the edge. Guillemot eggs are strikingly pyriform, tapering to a point, and highly variable in ground colour and scrawled markings.
The traditional explanation is that a pointed egg rolls in a tight arc rather than a straight line, returning toward its own position. That account has been challenged. Later work has argued that the shape instead improves stability by lowering the centre of gravity and increasing contact with the substrate, and that it also strengthens the egg or helps keep the blunt end clear of accumulated filth on a crowded ledge. The anti-rolling story survives, but not unqualified.
The individual markings serve a different job: adults recognise their own egg by its pattern, which matters when a bird returns to a ledge of near-identical neighbours. Chicks jump from the ledge at about three weeks old, before they can fly, and continue to be fed at sea by the male.