Cracking the guillemot’s egg

Everyone knows guillemots’ pointy eggs are shaped to roll in an arc to stop them falling from their cliff-ledge nests. Trouble is, it’s not true – they roll in too wide an arc. Last year, Tim Birkhead of the University of Sheffield found evidence that the shape is more about hygiene on guano-encrusted ledges ( BBC Wildlife, May 2017).

Now, Birkhead has published what he believes is a better explanation: “While doing fieldwork, the idea popped into my head that the shape of a guillemot egg would be more stable on a sloping ledge. Over 50 per cent of guillemot ledges are sloping. I tried it and it worked.”

Razorbills’ eggs, which are more classically egg-shaped, were far less secure on a slope. “More rigorous experiments confirmed that the more steeply sided the egg, the more likely it was to stay put.”

The shape might yet turn out to serve multiple functions: extra strength, as a defence against the impacts of crashlanding parents, included. And Birkhead’s not ruling out the hygiene hypothesis. “It’s clear that the blunt ends, containing the chicks, stay relatively clean.”

FIND OUT MORE

The Auk: Ornithological Advances https://doi. org/10.1642/AUK-18-38.1