Cracking The Mystery

Why is the guillemot egg so shaped? ‘To stop it rolling away’ is the textbook answer. But is it correct? Follow one man's mission to get to the bottom of an evolutionary mystery.

- Tim Birkhead

The unusually shaped eggs of the common guillemot are large, colourful and very nutritious. Three times the size of a hen’s egg, guillemot eggs were, for centuries, eagerly sought by those with access to their colonies. In some areas, it was thought that at the sight of approaching human egg-collectors (known as ‘eggers’), guillemots deliberately threw their eggs off the breeding ledges rather than allow them to be taken.

Nonsense, of course, but it was an explanation for the shower of eggs that invariably rained down from the guillemot’s breeding cliffs whenever humans disturbed them. Tourist steamers often blasted their horn or fired a shotgun at guillemot colonies, so their passengers could enjoy the spectacle of guillemots pouring off the cliffs, causing their eggs to roll off and smash onto the rocks below.

Observations like these seem to contradict the idea that their unusual shape – described as pyriform or pear-shaped, with one very pointed end – has evolved to prevent the guillemot’s egg from rolling off a cliff ledge, where it is incubated on bare rock.

Guillemot eggs were a prime target for eggers, not just because of their size, but because they could be collected in huge numbers from the typically very dense, very large colonies. Centuries after people began collecting and eating guillemot eggs, the question arose as to why they were an unusual shape, compared to those of most other birds.