Temperature drives the evolution and global distribution of avian eggshell colour

Abstract

The survival of a bird's egg depends upon its ability to stay within strict thermal limits. 9 Avian eggshell colours have long been considered a phenotype that can help them stay 10 within these thermal limits 1,2 , with dark eggs absorbing heat more rapidly than bright eggs. 11Although disputed 3,4 , evidence suggests that darker eggs do increase in temperature more 12 rapidly than lighter eggs, explaining why dark eggs are often considered as a cost to trade-13 off against crypsis [5][6][7] . Although studies have considered whether eggshell colours can confer 14 an adaptive benefit 4,6 , no study has demonstrated evidence that eggshell colours have 15 actually adapted for this function. This would require data spanning a wide phylogenetic 16 diversity of birds and a global spatial scale. Here we show evidence that darker and 17 browner eggs have indeed evolved in cold climes, and that the thermoregulatory advantage 18 for avian eggs is a stronger selective pressure in cold climates. Temperature alone 19 predicted more than 80% of the global variation in eggshell colour and luminance. These 20 patterns were directly related to avian nesting strategy, such that all relationships were 21 stronger when eggs were exposed to incident solar radiation. Our data provide strong 22 evidence that sunlight and nesting strategies are important selection pressures driving egg 23 pigment evolution through their role in thermoregulation. Moreover, our study advances 24 understanding of how traits have adapted to local temperatures, which is essential if we are 25 to understand how organisms will be impacted by global climate change. 26 27The impact of global climate patterns on the evolution and distribution of traits is an area of 28 increasing importance as global temperatures continue to rise. Birds' eggs are an ideal system for 29 2 exploring the intersection between climate and trait diversity, because a tight thermal range is 30 necessary for the survival of the developing embryo 8 , as eggs are unable to regulate their own 31 temperature 9 . As a result, many birds have adapted incubation behaviours and nest characteristics 32 in response to local conditions 10,11 . In addition to these behavioural adaptations, the adaptive 33 value of eggshell coloration for thermoregulation has been of longstanding interest 1,2,5 . These 34 eggshell colours are generated by just two pigments 12 and eggshell coloration is known to reflect 35 local environmental conditions 13 . 36The white colour found on many eggs (e.g., ostrich eggs) reflects incident solar radiation 37 from their surfaces, but can draw the attention of predators 14 . By contrast, dark brown or heavily 38 speckled eggs (e.g., artic loon eggs) may escape the visual detection of predators, particularly in 39 ground nesting birds 15 , but these darker eggs should heat more rapidly when left in the sun 1,16 . 40 Therefore, in hot climes the thermal costs must be balanced against the adaptive benefits of 41 cryptic pigmentation, while in cold climes thermoregula...

Global diversity and adaptations of avian eggshell thickness indices

“…The global diversity in eggshell traits for colour and patterning (Kilner 2006, Wisocki et al . 2019), stiffness (Hung et al . 2022), shape (Stoddard et al .…”

“…These studies provide strong evidence for factors underlying structural diversification of bird eggshells. Although largescale macroecological studies have examined egg shape (Stoddard et al 2017), maculation (Kilner 2006, Wisocki et al 2019) and cuticle structures (D'Alba et al 2016), other important eggshell factors are less well understood. This is particularly important given that eggshell traits are not mutually exclusive, and thus each needs to be integrated with other eggshell characteristics successfully to fulfil multiple purposes.…”

“…The global diversity in eggshell traits for colour and patterning (Kilner 2006, Wisocki et al 2019, stiffness (Hung et al 2022), shape (Stoddard et al 2017), size and structural components (D'Alba et al 2016) have been attributed to abiotic (humidity, temperature and altitude) and biotic (parent incubation strategies, nest structure and microbes) selective pressures that influence the survival and metabolic requirements of the developing embryo. For instance, egg shapes with a low surface area are more commonly found in hot, dry and sparsely vegetated environments to reduce water loss and exposure to UV radiation (Englert Duursma et al 2018).…”

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.…”

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

“…We accounted for the phylogenetic relatedness among birds by constructing a phylogenetic hypothesis using a sample of 9,999 fully resolved phylogenetic trees from a recent complete avian phylogeny 48,49 . Using these data, we calculated the Bayesian maximum credibility tree (Extended Data Fig.…”