For cliff-dwelling birds, pointy eggs are best

A study confirms that for cliff-nesting birds, pointy eggs are best.

Common murres or guillemots (Uria aalge) are small seabirds in the auk family that nest in colonies on steep rocky cliffs. The birds lay their eggs directly on the cliff ledge, without a protective nest.

The eggs have distinct pointed shape, which protects them from plunging into the ocean if they are laid on a sloping surface, or if a parent bird kicks them. Instead of travelling in a straight line, they roll in a circle, coming to rest where they began.

“Very little is known about how the murre egg shape affects its stability and viability in this setting,” says Mark Hauber, from the University of Illinois, US, who conducted the study with colleague Ian Hays.

The trademark pointiness and its very clear adaptive benefit to the survival of the murres and other members of the Uria genus have intrigued researchers since the 1980s. It was clear that the egg shape was keeping the eggs on the cliff, but the exact physics was hard to pin down.

Some early studies tried to unpack the problem by using model guillemot eggs made papier maché.

However, Hauber says, these studies failed to isolate specific features of the eggs – such as elongation, asymmetry and conicality – to robustly test the hypothesis.

Hauber and Hays were able to bypass the papier maché method and utilise a 3D printer to create eggs in specific shapes and sizes, enabling them to test each specific variable. In a series of experiments using video-recorded motion trials, dubbed the “egg-roll” tests, the pair was able to determine which factors were most likely to contribute to a static roll.

The findings, published in the Journal of Experimental Biology, confirmed previous hypotheses – that conical eggs are more likely to stay put. The key measure, they found, was the relationship between the resting angle of the egg – determined by its shape – and the steepness of the slope on which it lies.

“In general, an egg’s conicality was the most reliable predictor of its likelihood of staying put on inclined surfaces,” Hauber says. “This finding provides experimental support for natural selection shaping the unique form of murre eggs amongst all bird eggs.”