Variability in avian eggshell colour: A comparative study of museum eggshells

Background: The exceptional diversity of coloration found in avian eggshells has long fascinated biologists and inspired abroad range of adaptive hypotheses to explain its evolution. Three main impediments to understanding the variability ofeggshell appearance are: (1) the reliable quantification of the variation in eggshell colours; (2) its perception by birdsthemselves, and (3) its relation to avian phylogeny. Here we use an extensive museum collection to address these problemsdirectly, and to test how diversity in eggshell coloration is distributed among different phylogenetic levels of the class Aves.Methodology and Results: Spectrophotometric data on eggshell coloration were collected from a taxonomicallyrepresentative sample of 251 bird species to determine the change in reflectance across different wavelengths and thetaxonomic level where the variation resides. As many hypotheses for the evolution of eggshell coloration assume that eggcolours provide a communication signal for an avian receiver, we also modelled reflectance spectra of shell coloration forthe avian visual system. We found that a majority of species have eggs with similar background colour (long wavelengths)but that striking differences are just as likely to occur between congeners as between members of different families. Theregion of greatest variability in eggshell colour among closely related species coincided with the medium-wavelengthsensitive region around 500 nm.Conclusions: The majority of bird species share similar background eggshell colours, while the greatest variability amongspecies aligns with differences along a red-brown to blue axis that most likely corresponds with variation in the presenceand concentration of two tetrapyrrole pigments responsible for eggshell coloration. Additionally, our results confirmprevious findings of temporal changes in museum collections, and this will be of particular concern for studies testingintraspecific hypotheses relating temporal patterns to adaptation of eggshell colour. We suggest that future studiesinvestigating the phylogenetic association between the composition and concentration of eggshell pigments, and betweenthe evolutionary drivers and functional impacts of eggshell colour variability will be most rewarding.