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 longstanding problem in biology. One species whose pyriform egg has been the subject of some ‘explanations’ is the. For a long time, it was widely believed that the pyriform shape of Common Murre’s Uria aalge egg allowed it to either ‘spin-like-a–top’ or ’roll-in-an arc’, and thus reduce the risk of rolling off the breeding ledge. There is no evidence in support of either mechanism. Here, we present and test a new hypothesis: that the murre egg’s pyriform shape confers stability on the breeding ledge, thus reducing the chance that it will begin to roll. We test this hypothesis by measuring the stability of murre eggs of different shapes on slopes of 20°, 30° and 40° above the horizontal, and by comparing the stability of Common Murre eggs with those of the closely related Razorbill (Alca torda) whose egg is more elliptical. We show that more pyriform eggs are more stable, and easier to stabilize, on sloping surfaces. From a fitness perspective, the stability of the murre egg on a slope seems likely to confer an advantage and thus may be a strong force of natural selection favouring the pyriform shape.