Nest location and architecture as primary drivers of variation in UV reflectance in avian eggs

Abstract

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 embryos against solar exposure in open nests, whereas the egg detectability hypothesis posits that higher UV reflectance helps eggs stand out against the dark background of the nest, making them 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 signalling. 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. We did not find strong support for either of the two hypotheses across all birds. However, in two clades exhibiting notably high levels of UV reflectance (Passeriformes and Charadriiformes), species with higher UV reflectance values predominantly nest in open nests, suggesting a stronger effect of 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.

Keywords: UV reflectance; egg; eggshell colour; light exposure; macroecology; nest.

Figure 1.

Mollweide projection of the hue distribution of natural eggshell colour within the ultraviolet-sensitive avian colour space, with five representative eggs: (1) Chlidonias niger, (2) Horornis diphone, (3) Nyctidromus albicollis, (4) Melanerpes carolinus and (5) Tinamus major. Photos: Maria Elisa Mendiwelso.

Figure 2.

Maximum-likelihood estimates of ancestral states of the percentage of UV chroma reflected by avian eggshells. The scale bar indicates UV chroma from low values (whiter colours) to high values (purple colours). Changes along branches are calculated by interpolation [43]. The two sets of coloured circle symbol matrices at the end of each branch tip, the innermost circle corresponding to nest exposition and the outermost symbol corresponding to habitat density. We follow the taxonomy proposed by Clements et al. [49].

Figure 3.

(a) Scatter plot illustrating the interaction between UV reflection and mean brightness, with each point representing a species, grouped by nest exposure group (open, domed, cavity) using ellipses. Eggs placed in open and domed nest expositions had lower mean brightness and higher UV chroma compared with cavity nests. (b) Violin plot distribution of nest exposure and habitat density on a scale ranging from least exposed to most exposed, considering the mean value for each category.