Cracking The Mystery
BBC Wildlife|March 2021
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.

The first suggestions were made by an egg collector – not someone interested in consuming eggs, but rather in displaying them – William Hewitson. Writing in 1831, he described how, if knocked, a guillemot egg would rotate on its axis, “the larger end moving around the smaller in a circle keeping it in its original position”. What a great idea! And one with such intuitive appeal. But spectacularly wrong, for Hewitson’s idea was based on how one of his empty, blown guillemot eggshells would behave if set in motion, rather than an intact egg. But such was the appeal of Hewitson’s ‘insight’, that others, with little or no experience of guillemot eggs, elaborated and reinforced the idea, creating the spinning-like-a-top myth.

Hewitson’s idea is readily reinforced by a demonstration. Visit a museum, take a guillemot egg from a drawer, place it on a flat surface and give it a spin. Sure enough, it rotates like a top on its side, but remains in exactly the same place.

Yet as a demonstration of the significance of the guillemot’s pyriform egg, this is the worst of all experiments, and here’s why: select an egg – of any shape – of any other bird species from a museum drawer, or take a blown chicken egg, and try spinning it. They all spin, showing that their shape quite clearly has nothing to do with their ability to spin. What’s more, an intact egg with yolk or a developing chick inside will not spin, unless persuaded with considerable force. Spinning like a top is not the explanation for the guillemot’s pyriform egg.

A clutch of theories

Interestingly, some of those that collected guillemot eggs – the ‘climmers’ at Bempton, for example – who handled intact specimens, pooh-poohed the spinning-like-atop explanation, but were shouted down by the great 19-century popularisers, including Francis Orpen Morris, whose A History of British Birds was a bestseller.

Next in the roll-call of guillemot egg investigators was Russian biologist Lew Belopol’skii. Recognising that the spinning-like-a-top theory was nonsense, he began his own investigations in the 1940s, and came up with what, until recently, was the second most popular explanation for the guillemot egg shape. He demonstrated that a pyriform egg, placed on a flat, but gently sloping surface, would roll in an arc when released, and thus be prevented from disappearing over a cliff ledge.

Continue reading your story on the app

Continue reading your story in the magazine

MORE STORIES FROM BBC WILDLIFEView All

Theme park could go ahead despite site's protected status

Swanscombe peninsula is designated as an SSSI but campaigners fear it is not enough to save the wildlife haven from proposed development.

2 mins read
BBC Wildlife
Spring 2021

MIKE DILGER'S - WILDLIFE WATCHING

In his series of great places to watch wildlife in the UK, the star of BBC One’s The One Show this month heads to our pebble-strewn shores to spot the species that thrive on shingle beaches.

3 mins read
BBC Wildlife
Spring 2021

TRUTH OR FICTION? It's perfectly safe to airlift a rhinoceros

Flying rhinos upside down looks ungainly but is it harmful, too? New research set out to find the answer.

1 min read
BBC Wildlife
Spring 2021

Norway's star wolf moved for safety

Norway’s most famous wolf has been captured and moved south with a female companion to protect him from licensed killing.

1 min read
BBC Wildlife
Spring 2021

Helping hedgehogs

It may be the nation’s favourite mammal, but the hedgehog is rapidly vanishing from towns and countryside across the UK. Why have things gone so badly wrong for British hedgehogs? And, more importantly, what’s being done to help them?

10+ mins read
BBC Wildlife
Spring 2021

DISTURBANCE IN THE DEEP

With land resources rapidly depleting, eyes are turning to the seabed as a whole new source of metals. But at what cost to marine wildlife?

8 mins read
BBC Wildlife
Spring 2021

MEET THE SCIENTIST - Lauriane Suyin Chalmin-Pui

Well-being fellow, Royal Horticultural Society and postdoctoral researcher, University of Sheffield

2 mins read
BBC Wildlife
Spring 2021

Cranes becoming more common

Record numbers of common cranes bred in the UK last year, with 64 pairs producing 23 chicks.

1 min read
BBC Wildlife
Spring 2021

My Way Of Thinking - Mark Carwardine

The conservationist discusses Jair Bolsonaro’s actions concerning the Amazon rainforest and invites your thoughts on the subject.

3 mins read
BBC Wildlife
Spring 2021

Gorillas In The Midst Of A Pandemic

Close encounters with tourists may be exposing great apes to COVID-19.

2 mins read
BBC Wildlife
Spring 2021