For centuries, people have marvelled at the brown-flecked shell of a murre’s egg, which can range in color from a creamy white to a vibrant turqoise. But murre eggs have also fascinated people for another reason: their unusually angular tip, which is one of the pointiest in the avian kingdom.
In the 1800s, naturalists posited that this shape allowed the eggs to spin like a top in the wind, preventing them from rolling off the narrow ledges where murres nest. That was later disproved. Then, in the mid-1900s, the idea arose that the egg’s downward-angled tip helps it roll in a neat arc, just tight enough to stop it from tumbling into the breakers below. That theory has remained the dominant one and is still widely accepted today.
But British ornithologist Tim Birkhead has studied murres for 44 years, and he has doubts. “In all the time that these ideas had been kicking around, no one had seriously come up with any alternative explanations,” says Birkhead, professor of behavior and evolution at Sheffield University. Now, in two research papers, he’s not only debunked the prevailing rolling theory, but also has laid out his own ideas explaining the eggs’ unusual shape.
Part of the auk family, which has species around the world, murres lay their eggs on skinny rock ledges overlooking the sea. But despite the obvious risks, murres make good parents, says Birkhead, who has studied the birds mostly at Skomer Island off the coast of Wales in the UK. “In fact, very few eggs naturally fall off ledges.” He began to wonder, then, if the birds were so talented at keeping their eggs safe, how could the risk of falling be an evolutionary driver powerful enough to explain their odd shape?
In one of the two recent papers, Birkhead studied Common and Thick-billed Murres and came to the conclusion that the circumference of the egg’s rolling arc is also frequently bigger than the ledge on which murres nest. “I thought, if that isn’t the explanation, then what features of the [murre’s] biology are unique?”
From his years of observation, Birkhead winnowed it down to two features: space and cleanliness. The birds’ nesting ledges are typically cramped—up to 70 murres can inhabit a square meter—and consequently there’s a lot of trampling. Murres are also unwieldy fliers, which can cause them to crash-land onto other birds’ nearby nests. “So the first possibility was that the egg’s shape protects it from impact,” Birkhead says. Intriguingly, he discovered that compared to a typical oval egg, the shape of the murre’s egg places more of the shell in contact with the ground. That would dissipate the force of an impact across its surface, he hypothesized. Incidentally, the eggshell is also thicker in the part of the shell that touches the ground.
“The other feature of [murre] breeding ledges is that they are invariably filthy,” Birkhead says. “Sometimes it’s like going into a pigsty.” More than just a smelly inconvenience, the guano and dirt that encrusts an incubating egg can clog up the pores that allow chicks to breath and also cause infection.
Fortunately for murres, Birkhead discovered, the greatest concentration of pores is around the egg’s large, blunt end that is raised above the ground by the downward-angled tip. This protects it from the contaminated ledge. By surveying almost 100 eggs, Birkhead also found that murres had more pores at the blunt end compared to the Razorbill, their closest relative, whose nesting habitat is typically less toxic and grubby. That strengthens the idea that the pointed tip makes these eggs especially suited to the jam-packed nesting sites where murres breed. “Nobody else before had really noticed this business of the dirt being a major selective force on [murre] eggs,” Birkhead says.
So why has the rolling arc theory persisted for so long? Because people love an intuitive, simple explanation, Birkhead says—which, he’s also keen to point out, his hypothesis is not. “My guess is that unlike the rolling and the arc, there probably won’t be a single factor explanation. Most adaptations end up being compromises between different selection pressures.”
To the average person, pondering the shape of a bird’s egg in such depth may seem “utterly trivial," Birkhead says. But as he points out, understanding the purpose behind natural structures as well-adapted as the murre’s egg can have implications in fields ranging from mathematics to engineering. Not that Birkhead needs any justification for studying murre eggs—to him, they will always be endlessly fascinating. “I’m driven by scientific curiosity," he says. "It’s an interesting puzzle to try and solve."
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