Life on the edge: Are pointed eggs more resistant to breakage among cliff-breeding seabirds?

Credit: Blackartz

Why lay a pointed egg?​

The functional aspects of pyriform (pointed or pear-shaped) eggs laid by common guillemots, a large-bodied sea bird, have been of long-standing interest. Pyriform eggs are one of the most distinct in shape among birds – being extremely pointed at one end, similar to a cone.

Common guillemot eggs collected from Skomer Island. Credit: Marie Attard

Common guillemot eggs are incubated directly on bare rock within dense breeding groups, posing great physical risk to the egg. Their eggs can be easily knocked or trodden on during aggressive interactions between conspecifics and mated pairs or aerial attacks by predatory gulls. There is no  nest material to help cushion or support the egg from underneath, or prevent the egg from rolling off the cliff.

Given their extreme nesting environment, it is expected that common guillemot eggs are highly resistant to breakage, though this has never been quantified.

Common guillemot breeding colony. Credit: rwhgould

Are pyriform eggs particularly strong?

Pyriform eggs are unique in that they have a long near-flat surface between the equator and its pointed pole. This provides a stable position for the egg to incubate: the near-flat surface of the egg will typically stay in contact with the substrate over the incubation period. ​

Common guillemots have a large contact area between the shell and flat ground surface compared to razorbills. Although both species breed on the same cliff ledges, razorbills tend to incubate their eggs in crevices, while common guillemots will incubate their eggs on sloped cliff surfaces within dense breeding colonies. Credit: Marie Attard

For my research, I tested whether pyriform eggs are more resistant to breakage due to possessing a greater surface contact area with the ground.  

Using a 3D digital ‘crash test’ simulations (called Finite Element Analysis), I found that eggs with a larger contact area at the near-flat plane experience lower stress during incubation, and thus are likely more resistant to breakage. I also was able to break fresh guillemot eggs in the laboratory to find out how strong guillemot eggs really are. The pointed pole of the egg experienced the highest stiffness and breaking force values of any of the regions measured in this study, which is beneficial during laying pointed end first on a hard substrate. These findings are currently being prepared for publication. 

IN PREPARATION

Attard MRG, Holland C, Wroe S, Clausen P, Koeppel A and Birkhead T (In prep) Life on the edge: Evolution of eggs to resist breakage among cliff-breeding seabirds.