The point of a Guillemot's egg

Abstract

The adaptive significance of avian egg shape in birds is poorly understood. The pyriform (pear‐like) shape of the Common Guillemot's Uria aalge egg has long been considered to be an adaptation to prevent eggs rolling off the bare cliff ledges on which this species breeds. Rolling was thought to be prevented either by the egg spinning like a top, which is not the case, or by rolling in an arc, which it does but with little influence on whether the egg will fall from a ledge. We therefore sought alternative explanations for the pyriform shape of the Common Guillemot's egg. This species breeds in extremely dense colonies, which makes their eggs vulnerable to mechanical damage from conspecifics, and to contamination by debris such as faeces and soil. We present evidence consistent with both these possible explanations. First, the pyriform shape of Common Guillemot eggs means that a higher proportion of the eggshell lies in contact with the substrate and this may minimize the effect of impacts. Resistance to impacts may be further enhanced because their eggshells are especially thick where they are in contact with the substrate. Secondly, Common Guillemot eggs are often heavily contaminated with faecal material and other debris during incubation. Most contamination is on the pointed end of the egg where it is in contact with the substrate; the pyriform shape thus keeps the blunt end of the egg, which has the highest porosity, relatively free of contamination, which in turn may facilitate both gas exchange during incubation and the hatching process, because the chick emerges from the blunt end of the egg.

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

“…Empirical observations confirm that, among naturally incubated Common Murre eggs, fecal contamination of the eggshell is less likely at the blunt end of the egg (Birkhead et al 2017b).…”

“…In an earlier paper (Birkhead et al 2017b), we suggested that the pyriform shape might (1) reduce the chance of shell breakage by dissipating the forces of any impact-like an adult landing or stepping on the egg-across a wider surface of the shell; or (2) help to keep the blunt pole of the egg (where the embryo's head and the air cell are located) relatively free from debris and fecal contamination, allowing the embryo to respire more efficiently, because that region of the egg has a relatively high density of pores. Empirical observations confirm that, among naturally incubated Common Murre eggs, fecal contamination of the eggshell is less likely at the blunt end of the egg (Birkhead et al 2017b).…”

“…pear-like or pointed), and one of the most extreme is the pyriform egg of the Common Murre (Uria aalge; Birkhead 2017, Birkhead et al 2017a, 2017b, Stoddard et al 2017. The adaptive significance of this shape, and indeed of the shapes of all other birds' eggs, is unclear.…”

“…This idea was (and still is) widely reported in the ornithological literature (Drent 1975, Gill 2007, Lovette and Fitzpatrick 2016 and the popular press. But extensive testing by Ingold (1980) provided little conclusive support for this hypothesis (see also Birkhead 2017, Birkhead et al 2017a, 2017b.…”

Egg shape in the Common Guillemot Uria aalge and Brunnich’s Guillemot U. lomvia: not a rolling matter?

“…He also showed that parental behavior, including keeping the egg between their legs with the blunt end directed away from the bird, was important in keeping the egg on the ledge (Ingold 1980;see also Tschanz 1990;Ingold 2016). Elsewhere, we consider several other hypotheses for the pyriform shape of guillemot eggs (Birkhead et al 2017). …”