Living life on the edge: unique nano-structures on the guillemot eggshell assist with nesting in a harsh environment

Amongst vertebrates, the avian egg is unique in its structure and form. Bird eggs show a large range in shape, size and colour, the diversity of which has inspired much interest and debate. We have identified a new biological structure present in the surface of the eggshell, which is seemingly specific to one group of birds whose breeding ecology is unique within the avian kingdom. The guillemot group (Uria sp.) of the Alcidae family breed on exposed marine cliff faces, and with no nest used, the eggs are laid directly on the rock surface. The eggs are constantly exposed to sea-salt spray and guano from the incubating parents. The conical-like structural protrusions present in the eggshell surface of guillemot eggs are analogous with the nano-scale hierarchical arrangements identified in lotus leaves (Nelumbo sp.), the purpose of which is self cleaning. These structures are entirely absent in the eggs of 217 other birds species studied, including those nesting in similar environments, and those closely related to the guillemots from a phylogenetic perspective. Through comparative experiments we demonstrate how the guillemot eggshells exhibit hydrophobicity, have increased surface roughness and surface area, and a high rate of gas exchange under constant conditions. We conclude that these differences, a result of the conical protrusions in the eggshell surface, are an adaptation to the unusual breeding environment of the guillemot, in particular the salt and guana exposure