The adaptive significance of avian eggshell architecture

The avian eggshell plays a critical role in reproduction, providing the primary defence for the developing embryo against the outside world. The aim of this thesis is to understand how and why eggshell microstructure varies. I focus on one particular species, the common guillemot, which incubates its egg in a harsh environment – an exposed dirty, wet rock ledge – without a nest. I also make comparisons with other members of its family, the Alcids. Across Alcids, eggshell thickness increases with adult (parental) body mass. I find eggshells are thicker at the equator in Alcid species that lay more elongate eggs and those that incubate their egg(s) on rock. A putative consequence of enhanced shell thickness at the equator (and potentially pointed end) is that fewer pores are able to form there, and as a result, the thinner blunt end may require a higher pore density to satisfy gas exchange demands. In common guillemot eggs pore density is indeed negatively related to shell thickness. I find that total pore number relates to egg size – but not incubation period – across the Alcids. As eggshell pores are open channels from the outside into the egg, they pose a risk if they allow foreign matter to enter the eggshell. I show that shell accessory materials on common guillemot eggs provide protection, preventing foreign material from entering and blocking pores. I suggest that eggshells with a rough surface, particularly at the equator, may be better able to keep shell accessory material adhered to the shell, minimising the impact of abrasion from hard rock substrates. Intriguingly, I also show that surface microstructure relates to eggshell colour and pattern. Overall, these findings provide a detailed insight into how eggshell structure varies within and between individuals and species, with important implications for our understanding of avian eggshell function.