StatisticalAnalysisR

Two main hypotheses have been proposed to explain the function of ultraviolet (UV) reflection in avian eggs. The UV resistance hypothesis suggests that high UV reflectance protects against solar exposure in open nests, whereas the egg detectability hypothesis posits that higher UV reflectance helps to stand out against the dark background of the nest, making the eggs easier for parents to locate in enclosed nests. Therefore, eggshell reflection in the UV spectrum may serve multiple, possibly even opposing forces, including UV protection and visual signaling. We tested these two hypotheses using large-scale comparative analyses of eggshell UV reflection for over 500 avian species while considering the influence of various ecological, life history, and environmental traits associated with light exposure. Likewise, although no full support was found for either of the two hypotheses, two clades—passerines and Charadriiformes—exhibited notably high levels of UV reflectance. In both clades, species with higher UV reflectance values are predominantly nesting in open nests, supporting the UV resistance hypothesis. This research contributes to a deeper understanding of the mechanisms driving UV coloration in avian eggs and unravels the interplay between life history traits potentially associated with UV reflectance in specific clades under variable nesting conditions.