Crack­ing the guille­mot’s egg

BBC Wildlife Magazine - - Wild News -

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

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