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, Professor 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 crash-landing parents, included.
And Birkhead’s not ruling out the hygiene hypothesis. “It’s clear that the blunt ends, containing the chicks, stay relatively clean.”
Read the full paper in The Auk: Ornithological Advances.