Explain why the eggs of some seabirds are pear-shaped

According to the journal Auk: Ornithological Advances, British scientists have explained why the eggs of some seabirds are pear-shaped.

Half a century of research has shown that this form makes seabird eggs difficult to break and prevents them from accidentally rolling off the cliff into the sea.

Explain why the eggs of some seabirds are pear-shaped
The pear-shaped eggs of some seabirds appeared in a centuries-long evolution – (Photo: Flickr).

The eggs of some Alcidae birds are unusually pear-shaped, with one end slightly pointed, while the other end, on the contrary, is wider than the eggs of other birds. Alcidae live on a rocky shoreline and lay their eggs directly on bare rocks.

Previously, ornithologists thought that this form of eggs does not allow them to fall off cliffs and that if the bird accidentally pushes the egg, when it falls, the egg will not roll in a straight line, but in a circle. But now scientists from the University of Sheffield (UK) say that this is not entirely true.

According to the results of the study, the problem is not the trajectory of the movement, but the mechanical stability : the irregular shape of the egg does not even allow it to roll down. Because of the bulge under the eggs, it is completely difficult for the eggs to roll out of the crater.

This form, according to the authors, emerged during a centuries-long evolutionary process, as the more pear-shaped eggs had no chance of falling into the sea and were more likely to survive until hatching into young birds.

To test the hypothesis, the authors tested eggs with different shapes when stabilized on an inclined surface. Scientists compared the eggs of the pointed-billed Uria bird (Uria aalge) and the albatross bird (Alca torda). At different inclinations of the surface from 20° to 40°, the pear-shaped egg was shown to be more stable. Furthermore, pear-shaped ones are harder to push down.