Places – Arctic II

Brünnich’s guillemots, also known as thick-billed murres, are in the auk family that breeds in large colonies across the Arctic. They are black-and-white birds with a thick, short bill and a white line along the bill’s base.

They nest on steep coastal cliffs, shoulder-to-shoulder with virtually no nest structure, relying on strong parental coordination to prevent eggs from falling. They have one egg per pair, laid directly on bare rock.

Their food is primarily fish (like polar cod and capelin), but also crustaceans. They can dive over 150 meters deep, and can stay submerged up to 4 minutes, using their wings to ‘fly’ underwater.

The egg’s colour varies from pale blue to turquoise and may be blotched or streaked. One idea is that the blue pigment, biliverdin, may have antioxidant or antimicrobial properties, helping to protect the egg in unsanitary conditions typical of guillemot colonies. Each egg has a unique pattern, and in dense colonies with thousands of birds, a bright egg with a specific pattern is more easily recognised by the parent, especially when returning from sea. It’s also worth noting that guillemot eggs are unusually pointed at one end. This causes them to roll in tight circles rather than off the cliff, a vital adaptation for exposed nesting ledges.

It’s interesting that other cliff-nesting seabirds like razorbills and some kittiwakes also lay more pointed or pyriform eggs. However, only birds that nest on narrow or exposed ledges without building a nest have evolved this adaptation. It’s logical to assume that egg shape must reflect some kind of trade-off between nesting environment, clutch size, body shape, and flight adaptation. So in this case the most important “constraint” was trying to keep the egg on the ledge.

You may ask, why didn’t guillemots just nest somewhere else, rather than evolve weirdly shaped eggs to cope with dangerous cliffs? The answer is just as simple, because nesting on cliffs gives them massive survival advantages that outweigh the risks. The idea is that they avoid ground predators such as arctic foxes, and they don’t have to compete with other birds for nesting places. In addition they can nest shoulder-to-shoulder, thousands strong, maximising communal protection and space use. And don’t forget sea cliffs usually border productive oceans. This makes feeding chicks much easier. The evolutionary downside to “nesting” on vertical rocks, is that nesting materials are scarce on cliffs, and even, once built, would be vulnerable to wind. The answer was to stay of a sheer rock face, not build a nest, and evolve a shell that didn’t roll off.

One egg means there is still space on the rock to protect it, and both male and female guillemots take turns incubating their precious single egg. In fact the egg is held upright, pointed end down, balanced on the bird’s feet and under its belly feathers. After hatching, one parent usually stays behind to guard the chick from gulls and other threats while the other hunts for food to bring back.

There is also a very specific “cliff jump” culture. Around 20 days old, while still flightless and fluffy, the chick leaps off the cliff to the sea below. This is done at night, often with the father calling from the water below. The male guillemot then guides and feeds the chick at sea for several more weeks, and the female often no long participates in “child care”.

As seen in the photos and video, guillemots exhibit one of the most crowded breeding systems of any bird species on Earth. Each pair claims a tiny spot, often just 10–15 cm wide, sometimes even touching their neighbours. There is no physical boundary or nest, just a strong sense of site fidelity, and each bird returns to the exact same spot each year. Amazingly they recognise their specific nesting spot, mate, and neighbour pairs.

Colonies often exhibit synchronised breeding, incubation, and chick-rearing. This timing reduces predator impact by overwhelming predators with too many targets at once (a strategy called predator swamping).