Guillemot

Cliff colonies

In summer, guillemots nest in vast, noisy, jostling colonies on narrow ledges of sea cliffs and rocky stacks — sometimes packing 20 birds per square metre. There is no nest: eggs are laid directly on bare rock. The sheer density means incubating birds must be tolerant of constant jostling by neighbours. Britain and Ireland hold some of the largest guillemot colonies in the world, at Bempton Cliffs, Hermaness, and the Bass Rock.

Submarine flight

Guillemots are birds of two worlds: on land, they are awkward and upright; in the sea, they are supreme. They use their wings to “fly” underwater, reaching depths of 180 m in pursuit of fish (primarily sand eels, sprats, and small herring). Their wings are a compromise between aerial flight and underwater propulsion — short and stiff enough for diving, but just long enough to keep the birds aloft in the air.

The pyriform egg

The guillemot’s egg is famously pear-shaped (pyriform). If knocked, it rolls in a tight circle rather than rolling in a straight line off the cliff edge. Whether this shape evolved as an adaptation to reduce egg loss on cliff ledges, or simply develops due to the physical stresses of laying, is still debated by ornithologists.

Bridled form

Some guillemots have a white eye-ring and line running behind the eye, giving a “bridled” appearance. This is a genetic variant rather than a separate subspecies, and the proportion of bridled birds increases toward the north — from almost none in Iberia to over 50% in Iceland.