Egg Shape in Birds

Birds demonstrate incredible intra- and interspecific diversity in various egg traits. In particular, there is considerable diversity in egg shape across bird species from nearly spherical to extremely pyriform (pointed) eggs. This variation in avian egg shape has long fascinated researchers and numerous efforts have been made to quantify egg shape traits and examine the possible evolutionary drivers and functions of the observed patterns of avian egg diversity. Previous studies have attempted to examine egg shape at various taxonomic scales and the evidence from these studies collectively suggest that avian egg shape diversity might be driven by two main sources: (i) indirect selection via anatomical constraints or life history trait changes, and/or (ii) direct adaptive selection during the incubation period. However, it is still unclear as to how important these drivers of egg shape diversity are and how their importance might alter at different taxonomic scales. Furthermore, current studies on avian egg shape lack a universally agreed method of accurately quantifying all egg shapes, which makes comparing studies and assimilating overall patterns of egg shape diversity challenging. This thesis presents published work that aims to contribute further to the understanding of avian egg shape diversity. Firstly, the thesis presents a paper highlighting a novel automated image analysis method that more accurately quantifies egg shape, compared to previous modelling methods, that could be used by all future studies. Second, the thesis presents several papers examining the adaptive function of the extremely pyriform egg of the Common Guillemot (Uria aalge). In these papers, evidence is presented that brings in to question the plausibility of the previously popular ‘rolling-in-an-arc’ adaptive explanation. Instead, three new alternative adaptive hypotheses for the Common Guillemot’s pyriform egg are presented: (i) mechanical damage prevention/limitation; (ii) faecal-debris contamination limitation, and (iii) increased stability. Based on current available evidence, it is concluded that the stability hypothesis is currently the strongest supported adaptive explanation for the Common Guillemot’s pyriform egg shape. Thirdly, the thesis presents a published paper that looked to examine potential drivers of egg shape patterns at a broader taxonomic scale. Here, the paper examined patterns of egg shape in alcids (Alcidae) and penguins (Spheniscidae), two taxa containing species with considerable variations in egg shape and incubation environments and behaviours. In this study, evidence is presented that highlights the importance of incubation site characteristics as a driver of egg shape variation, something not evident in some other studies at broader taxonomic scales.