The common guillemot (Uria aalge), also known as the common murre in North America, is a large auk. It spends most of its time at sea, only coming to land to breed on rocky cliff shores or islands. Dark brown and white, not as black as the similar razorbill, it has a 'bridled' form with a white ring round the eye and stripe behind it. Guillemots breed in colonies at high densities, nesting pairs may be in bodily contact with their neighbours. They make no nest, their single egg is incubated on a They lay their pear-shaped eggs without the shelter of a nest directly on to a bare rock ledge on a cliff face. The centre of gravity is far from the centre of the egg. When the egg starts to roll on the sloping rock, it goes into a curved course and returns all by itself to a position of equilibrium. By its own dynamics, the egg protects itself from falling off.