Data from: The pyriform egg of the Common Murre (Uria aalge) is more stable on sloping surfaces

The adaptive significance of avian egg shape is a long-standing problem in
biology. For many years, it was widely believed that the pyriform shape of
the Common Murre (Uria aalge) egg allowed it to either “spin like a top”
or “roll in an arc,” thereby reducing its risk of rolling off the breeding
ledge. There is no evidence in support of either mechanism. Two recent
alternative hypotheses suggest that a pyriform egg confers mechanical
strength and minimizes the risk of dirt contamination of the blunt end. We
present a new hypothesis: that the Common Murre egg's pyriform shape
confers stability on the breeding ledge, thus reducing the chance that it
will begin to roll. We tested this hypothesis by measuring the stability
of Common Murre and Razorbill (Alca torda) eggs of different shapes on
slopes of 20°, 30°, and 40° above the horizontal. Common Murre eggs were
more stable, and easier to stabilize, than the more elliptical Razorbill
eggs. Within Common Murre eggs, more pyriform eggs were more stable. From
a fitness perspective, the stability of the Common Murre egg on a slope
seems likely to confer an advantage and thus may be a strong force of
natural selection favoring the pyriform shape.