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

TL;DR

This study explores how bird eggshell surface textures vary across species and how these textures relate to nesting environments and egg appearance.

Contribution

The paper introduces a novel phylogenetic analysis of eggshell texture using optical profilometry across a large avian dataset.

Findings

Species laying immaculate eggs have smoother eggshells compared to those with patterned eggs.

Eggshells from semi-enclosed nests are smoother than those from exposed nests.

Most species have eggshell surfaces dominated by valleys rather than peaks, based on surface skewness.

Abstract

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 (Sa, nm), surface skewness (Ssk) and surface kurtosis (Sku), which describe different aspects of the properties of eggshell surface texture. Sa represents the average height variations on the surface, providing a measure of smoothness or roughness. In contrast, Ssk reveals the distribution of surface features, where positive values signify a predominance of peaks, while negative values indicate a greater presence of valleys. Lastly, Sku 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 Ssk. By exploring the properties and performance of porous surfaces in nature, we may inspire future biomimicry designs that take advantage of these discoveries.

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