Scratching beyond the surface: examining macroecological patterns in avian eggshell texture

The surface texture of bird eggshells differs remarkably between species and is thought to play a substantial role in providing physical and microbial protection for the developing embryo. We used high-resolution optical profilometry to establish the key evolutionary drivers of surface textural diversity in eggshells from 453 bird species across 98 families. Within a phylogenetically informed framework, we aimed to determine which life-history traits and nesting environments probably determine eggshell surface texture. We measured surface roughness (S_a, nm), surface skewness (S_sk) and surface kurtosis (S_ku), which describe different aspects of the properties of eggshell surface texture. S_a represents the average height variations on the surface, providing a measure of smoothness or roughness. In contrast, S_sk reveals the distribution of surface features, where positive values signify a predominance of peaks, while negative values indicate a greater presence of valleys. Lastly, S_ku assesses the geometry of these features, with values exceeding 3 suggesting the presence of sharp peaks or deep troughs, and values below 3 indicating a flatter, more uniform surface. Overall, eggshell surfaces were smoother among species that lay immaculate eggs, meaning those without any pattern, in contrast to maculate eggs. Eggshells from semi-enclosed nests had smoother surfaces than those laid in exposed (cup, bowl, platform, no nest) nests. We found that 90.1% of the species had eggshell surfaces mainly composed of valleys rather than peaks, based on their S_sk. By exploring the properties and performance of porous surfaces in nature, we may inspire future biomimicry designs that take advantage of these discoveries.