The global distribution of avian eggshell colours suggest a thermoregulatory benefit of darker pigmentation

Abstract

The survival of a bird's egg depends upon its ability to stay within strict thermal limits.Avian eggshell colours have long been considered a phenotype that can help them stay within these thermal limits 1,2 , with dark eggs absorbing heat more rapidly than bright eggs.Although disputed 3,4 , evidence suggests that darker eggs do increase in temperature more rapidly than lighter eggs, explaining why dark eggs are often considered as a cost to tradeoff against crypsis [5][6][7] . Although studies have considered whether eggshell colours can confer an adaptive benefit 4,6 , no study has demonstrated evidence that eggshell colours have actually adapted for this function. This would require data spanning a wide phylogenetic diversity of birds and a global spatial scale. Here we show evidence that darker and browner eggs have indeed evolved in cold climes, and that the thermoregulatory advantage for avian eggs is a stronger selective pressure in cold climates. Temperature alone predicted more than 80% of the global variation in eggshell colour and luminance. These patterns were directly related to avian nesting strategy, such that all relationships were stronger when eggs were exposed to incident solar radiation. Our data provide strong evidence that sunlight and nesting strategies are important selection pressures driving egg pigment evolution through their role in thermoregulation. Moreover, our study advances understanding of how traits have adapted to local temperatures, which is essential if we are to understand how organisms will be impacted by global climate change.The impact of global climate patterns on the evolution and distribution of traits is an area of increasing importance as global temperatures continue to rise. Birds' eggs are an ideal system for certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.

A single nucleotide polymorphism variant located in the cis‐regulatory region of the ABCG2 gene is associated with mallard egg colour

“…The eggshell pigments also serve a thermoregulatory role to maintain normal embryonic body growth (Westmoreland et al, 2010). Birds living in cold habitats, particularly those birds with nests that are exposed to incident solar radiation, have darker eggs, which can heat up more rapidly than more lightly coloured eggs when they are exposed to solar radiation (Wisocki et al, 2020). Furthermore, eggshell coloration may result in crypsis or mimetism; cryptic eggs can remain undetected by predators, whereas brood parasites may mimic the coloration of host eggs (Kilner, 2006; Sullivan et al, 2003).…”

Hosts elevate either within-clutch consistency or between-clutch distinctiveness of egg phenotypes in defence against brood parasites

“…Why are some host species consistent in their egg phenotypes, and others (combinatorially) distinct? Which defence is elevated by coevolution with parasites may be influenced by selection from ecological factors aside from brood parasitism: certain egg phenotypes may be costly with respect to thermoregulation, protection from UV radiation, or camouflage [54][55][56][57], and increased susceptibility to host colonizations by other species or host races of the parasite [29,38], potentially limiting distinctiveness. Moreover, non-adaptive factors may also mean that the null hypothesis is not necessarily that both consistency and distinctiveness should be low in the absence of parasitism.…”

Ecological drivers of eggshell wettability in birds

“…Global variation in multiple avian eggshell surface properties (e.g. wettability, eggshell colour and luminescence [91]) is strongly predicted by the temperature of a species' breeding range. Climate-driving changes in breeding performance across birds are likely to be influenced by these traits, and species that persist in future climates may be able to do so in part owing to the adaptability of their eggshells to environmental challenges.…”